I am reporting back with my findings after experimenting with some parts. As far as I know, these findings apply to High Speed, Grand Lizard, Road Kings, and Pinbot.
The backbox speaker is a standard 4"x10" speaker that is commonly found in automotive. The only trick is finding one that is flush with the mounting surface - most speakers have a midrange/tweeter section that sticks out above the frame. Since the 4x10 speaker mounting surface is a wood piece with slot holes cut through it to act as the "grill", your options are:
1) Find a flush-mount 4"x10" speaker (Pyle PLX4102 verified by badpenny61 to be flush mount)
2) Find a regular 4"x10" and cut out the wooden "grill" area. Apply new speaker grill cloth (such as amazon.com link ») if you butchered the existing speaker cloth by accident.
The cabinet woofer is a standard 6.5" speaker so there are many options. The main constraint is to buy a speaker with a mounting depth no more than approximately 2 3/4", at which point the speaker would rub against the playfield top edge in its raised position. One could also buy a full-range speaker rather than a woofer although the higher frequencies would certainly be pointless. Also, all else equal, a speaker optimized for bass will put out better low frequency performance than a speaker built to handle bass and mid-range frequencies such as in a typical full-range coaxial. I used a Pyramid WX65X and it just barely rubs against the top of the playfield in its raised position.
I recommend using crimp-on tab connectors to mate the wire to the terminals on the speaker (such as http://www.parts-express.com/pe/showdetl.cfm?partnumber=095-282). The reason is that, although any polarity will work, it is worth experimenting with the polarity on the cabinet speaker. Swap the two wires and listen - the set up with the most bass (when working in conjunction with the cabinet speaker) is the "correct" hook up.
HIGH PASS FILTER
A high-pass filter ("bass blocker") is probably a good idea for the backbox speaker as it will never make good bass with an unsealed backglass as part of the speaker cabinet. Low frequencies will only distort the backbox speaker. I used Radio Shack 220µF 35V 20% Axial-Lead Electrolytic Capacitor Model: 272-1017 (https://www.radioshack.com/product/index.jsp?productId=2102485) which for a 4 ohm speaker should result in a high pass frequency of 180 Hz. (See calculator here: http://www.apicsllc.com/apics/Misc/filter2.html) Keep in mind that this is a very mild filter - only 6dB of attenuation per octave. But since 3dB is a doubling of power to a speaker, this is a real benefit.
LOW PASS FILTER
A low-pass filter for the cabinet speaker is strictly optional if you think it lends a better sound blend - high frequencies will not significantly distort a woofer.
Although the setup can handle 4ohm speakers for both locations, it is best to make sure both the backbox and the cabinet are rated for the same impedance. The reason is that, without separate volume controls, the speaker with high impedance is more likely to have a more muted volume. You may use this to your advantage, though - if you can find an 8ohm 4x10 speaker and a 4ohm 6.5 woofer, the setup could be a bit heavier in bass.
I highly recommend a volume control (really an attenuator) for the backbox speaker. Because most people like more bass, reducing the volume on the backbox speaker is a good way to increase the relative volume of the cabinet speaker that has been presumably upgraded to a bass-capable speaker. Key words are "L pad attenuator". I purchased this one: amazon.com link »
Use the following wiring diagram (taken from http://techtalk.parts-express.com/showthread.php?205712-L-pad-wiring ). Don't worry too much about the polarity. As long as 1 and 3 go to the amplifier somehow and 1 and 2 go to the speaker somehow, you are fine.
If you want to be "correct", use the pin-out below.
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If you combine a high-pass with the L-pad, my preference has been to insert the high pass capacitor between the L-pad and the speaker. Again, the primary value in a 1st order low pass (as you would get with a single in-line capacitor) is hypothetical more than audible to my ears. I put it between pin 2 of the L-pad and the speaker.