I think I've seen clay wire-in a second leafswitch on the start button so that when you press the start button it would also add a credit.

I have also seen people make the replay value very low so that most of the time on the first ball you score a replay credit which also has the same effect.

You can patch the ROM to add a free play setting - Oliver did this a few years ago.

http://www.pinball4you.ch/okaegi/pro_soft.html

I've done this on a MMPM and it works fine - no need to hack the cabinet.

viperrwk

Late 70's, early 80's depending on the manufacturer.

You can set the first threshold for winning a game very low so it always rewards you.
You can mount a switch behind the coin return button so that when you hit coin return you add a credit.
You can mount a second isolated switch behind the start button so that when you hit start button you add a credit as you start the game.
You can rewire the coin door slam switch with the wires to a coin slot so every time you want to add a credit you thump the door.
On Gottlieb System 1 and 80's you could place a jumper on the diode board across the coin slot credit switch and the start switch so every time you start a game, you add a credit. (Preferred as there is no learning curve for guests and the game thresholds match the card on the apron)

Note that none of these options involves taking a drill to the front of your machine.

I really like the "first threshold" option for four reasons:

1. It forces you to get a battery pack (or similar game save solution) installed and working. It's an important step in keeping early Bally pins in "ready to play and go anywhere" shape.
2. It lets you know when your batteries die and need replaced. (No longer holds the threshold setting.)
3. It fires the knocker to scare the crap out of players and also confirms that it is working.
4. It is the least intrusive to perform. All it requires is adding a game settings save solution (which every game should have anyway) and then setting the first replay level to like 20,000 pts.

One thing to also mention is that on MOST Bally pins, you must also set the DIPS on the main board to award a "CREDIT" or "REPLAY" award, instead of pts. OR an extra ball when that first scoring threshold is achieved.