This pretty easy DIY mod lets you control the center lockdown bar action button with a piano sustain pedal. This is good for games like Black Knight Sword of Rage where the Magna save is essentially rendered unusable by taking it away from the flipper buttons like prior entries. It also can be used to make the Heat Ray charge (or any action button functions) of Godzilla usable while still keeping both hands on the flippers. Total cost should be a little over $30. I have tested this with these specific parts. You're free to mix it up, but for the best chance of success, just get the parts I have laid out here with pictures and links.

This is specifically for the action button, but it would be trivial to adapt this to work with the flipper buttons, too, so if a person has limited use of one hand or the other (or both!) the pedal could handle the function for that flipper, allowing them to play after adapting to working the flippers with their feet.

This also would work for JJP Guns n Roses to add a pedal to select patches and cancel album mode with your foot. The main difference would be the switch bracket mounting is a little different, but everything else is essentially the same.

Parts you need:
Cable Extension:
amazon.com link »
02-extension.jpg

CASIO SP-3 Pedal
NOTE: THIS SPECIFIC PEDAL is the one I recommend. Sustain pedals have two polarity versions. One is the switch is always on, and pressing it interrupts that, the other has the switch off and pressing the pedal engages it. This casio is the "switch off at rest" kind needed for this project. Others, like the Yamaha FC-5 are the reverse and will not work without modification. A universal pedal may work because you can select the type with a switch on the pedal, but I didn't test that. If you want hassle-free DIY on this, just get this pedal.

amazon.com link »
01-pedal.jpg

Cable Clamps (you only need ONE, and if you have that on-hand, no need to buy a package of them here):
amazon.com link »

You will also need (and probably have these already):
2 x small zip ties
1 x hex head 1/2" wood screw

And these tools:
Soldering iron and a small amount of solder
Socket driver
1/4" Socket
5/16" Socket
11/32" Socket

Turn off the machine
Remove the lockdown bar and set aside.
Remove glass and set aside (if you're on cement, put something between the glass and cement so as not to trigger a thermal explosion of the safety glass from the temp differential between the glass and cement).
Remove balls and set aside
Lift Playfield
Take cable extension and cut off the MALE SIDE. Be very careful NOT to cut the receptacle/FEMALE side.
Carefully cut the into the insulation and pull it back about 1.5"-2" on the newly cut side
03-wire-end.jpg
Unwind the foil wrapping and copper shielding wires and cut both of them off at the point where they go into the insulation.
CAREFULLY (they're fine gauge) strip about 1/4" insulation off the red and white wires
Set now-prepared cable aside
04-wires-trimmed.jpg
The speaker in the cabinet has 4 x 11/32" nuts holding it down
05-speaker.jpg
Take a socket driver and 11/32" socket and remove the 4 nuts and set nuts aside
06-speaker-unscrewed.jpg
Lift off the speaker and set aside
07-speaker-removed.jpg
Lift the speaker screen on the three posts shown and rotate it, exposing the cutout
08-speaker-removed.jpg
Turn on your soldering iron here so it can heat up while you do the next step
The Action button light and switch are on a bracket attached with two 1/4" hex head wood screws and a 5/16" nut
09-action-bracket.jpg
Take the socket driver and attach a 1/4" socket. Remove the two 1/4" hex head wood screws and set aside by the speaker nuts
Change the socket on the driver to 5/16" and remove the bracket nut. Note that the screw it's attached to holds the coin door at the top, so don't lose the screw on the other side that may try to fall out.
Set wood screws and nut you removed aside with the speaker nuts.
10-screws.jpg
Bring the bracket around and rest it on the lockdown receiver ledge the two wires we're going to add to are on the left side of the switch blades
Use a small amount of solder to tin the ends of the two wires you're going to attach.
11-solder points.jpg
Attach one wire to each connection point as shown, making sure that you don't release the existing wires when you melt the solder.
12-soldering-complete.jpg

--- CHECK YOUR WORK HERE ---
Drop the action button bracket down and let it hang. Plug the pedal into the extension cord through the coin door, drop the playfield to the service position and turn the machine on.
When it's done booting, with the coin door open, pull the white post on the inside left of the coin door frame to activate the switches.
Bring up the service menu and go into switch test mode.
13-menu-progression.jpg
Once in that mode, press the pedal. If your work is ok, it should REGISTER a small square on the diag screen when you PRESS the pedal, and then the box should DISAPPEAR when you RELEASE the pedal.
14-pedal-pressed.jpg
15-pedal-released.jpg

If this is NOT the case, turn the machine off and check your work. DO NOT PROCEED until this test WORKS.

Once the test works, push the white post back in, unplug the pedal from the extension, then the machine off and raise the playfield again.

Bring the action button bracket back up and zip tie the wires coming off the switch, as close to the cut insulation of the extension as practical. The zip tie should have the two wires of the switch and the insulated, shielded extension wire in it.
16-first-zip-tie.jpg
Return the action button bracket under the lockdown receiver and re-install the 2 wood screws and one 5/16" nut.
Zip tie the insulated extension to the shrink-wrapped wire bundle (as shown) and then add the extension to the wire clip on the inside of the coin door. It's a tight fit, but a flat screwdriver may help open the clip easier so you can slip the extension wire in as shown. Feed the wire through the larger clip just above the coin door power cutoff switch and then feed it through the other 3 wire clips on the inside left side of the cabinet.
17-second-tie-and-routing.jpg
Grab one cable clamp and put it around the extension wire, near the female receptacle the pedal will plug into.
18-secure-plug.jpg
NOTE:I mounted this in the well here so if the cable is unplugged and the legs removed, the plug will be tucked away and not dragging. The height of the plug and depth of the speaker well are perfectly matched. Make sure the female receptacle is pointing to the FRONT of the machine.

Use the 1/4" socket and socket driver to screw the screw into the clamp with the screw below and the clamp above.
19-clamp-screw-placement.jpg
The bend of the clamp on top should be flush with the speaker cutout so it doesn't interfere with the speaker screen when you reinstall it.

Reinstall speaker screen making sure the extension wire is routed on the side of the screen as-shown and not pinched too hard.
20-replace-screen-route-wire.jpg
Reinstall speaker onto posts as shown and reinstall nuts.
21-replace-speaker.jpg
DO NOT OVERTIGHTEN the nuts on the left side.
22-replace-speaker-post-nuts.jpg

Plug the CASIO pedal into the female receptacle you just installed under the machine.

Lower playfield.
Return balls to trough.
Carefully reinstall glass.
Re-attach lockdown bar (and snap locks to keep it on).
Turn on machine
When bootup finishes, test the pedal in switch test mode as you did before.
If it works, you're done. Exit the menu, close the coin door and enjoy the choice of hands-on (lockdown bar) or hands-free (pedal) button activations - both work!

23-pedal-pic.jpg

All done with this project? Here's a couple other DIY guides I've made for newer Spike machines:

Are LED lights popping out of your spotlights (not just on Spike machines!)? Make sure they never fall out again:
https://pinside.com/pinball/forum/topic/fixing-led-bulbs-flashers-that-fall-out-of-spotlights#post-4402506

How about a pool noodle to protect your playfield glass when it's off the machine:
https://pinside.com/pinball/forum/topic/pool-noodle-protector-for-playfield-glass-when-off-the-machine#post-6961597

Have a Stern machine from KISS or after with the softer wood cabinet? Reinforce it to prevent a splitting cab:
https://pinside.com/pinball/forum/topic/reinforcing-stern-cabinet-legs-step-by-step

Have an $80 alibaba hologram fan topper you want to cleanly power so it turns on and off with the machine? Do this:
https://pinside.com/pinball/forum/topic/hologram-fan-or-lightedpinballmods-power-fix#post-6241182

Are your Iron Maiden speakers crackling when you crank it up? Fix that with a cheap external amp:
https://pinside.com/pinball/forum/topic/replacing-on-board-amp-with-external-amp-for-iron-maiden

Add a working lockdown bar button to your Iron Maiden
https://pinside.com/pinball/forum/topic/iron-maiden-how-to-add-a-working-lockdown-bar-button

Deadpool Katana entrance protection:
https://pinside.com/pinball/forum/topic/protecting-katana-lane-entrance-from-playfield-damage

Is your Spike machine occasionally resetting during hectic play, torpedoing your high score runs? This guide to fixing it might be for you:
https://pinside.com/pinball/forum/topic/fixing-stern-reboots-on-spike

Replace the noisy power supply fan in Spike machines with a quiet one:
https://pinside.com/pinball/forum/topic/replacing-stern-spike-noisy-ps-fans

Do you have a WoZ ECLE and think it's too loud in a quiet room? Replace two fans and make it almost silent:
https://pinside.com/pinball/forum/topic/quieting-down-a-woz-ecle-machine

And here are some other plug and play mods you can just buy:
Iron Maiden Light Tree Mod for colored mode lights:
https://pinside.com/pinball/forum/topic/iron-maiden-colored-mode-lens-tree-mod

Metallica F-U-E-L lights bracket:
https://pinside.com/pinball/forum/topic/metallica-f-u-e-l-gauge-lens-bracket

ColorDMD gasket for Stern machines with the angled speaker panel:
https://pinside.com/pinball/forum/topic/colordmd-gasket-for-angled-stern-speaker-panels

Great! Post back and let me know how it ends up working out for your family.

The instructions definitely tell you to cut off the foil and copper wiring (that are shielding only) sticking out, but I made it more clear. Thanks for the feedback.

Are you posting video of the install or video of using it?

I'm actually pretty excited to try one for GNR to change badges and cancel album modes.

I never had much problem with that one, except for having enough time to glance up to see when the fuse was IN the sweet spot, but the pedal isn't helping me with that.

It's just a little more work because the wires on the switch on GnR are shrink wrapped. You have to cut the shrink wrap to access the existing solder points and add the wires from the pedal.
switch-compare.jpg

You also need to feed the naked wire end up behind the lockdown plate (following the path of the switch wire) and BEHIND the metal bracket the switch mounts to BEFORE soldering it. Don't ask me how I learned this is the correct order of operations.
feed wire (resized).jpg

I have more GnR install pics and will be updating the other thread I made for the pedal install process on Spike with the bit that's specific to GnR, but the rest of the install is pretty much the same.

No just the red and white wires stripped and exposed on the ends so you can solder them. The copper wire and foil paper are just shielding. You cut that off for the exposed part.