I think this can be done using a microcontroller, something like the
Arduino.
First, the design concept:
Take your 1P pushbutton, and connect it to one of your microcontroller inputs. Hook one of the microcontroller outputs to your iPAC 1P port. The microcontroller program flow would be something like this:
1P is pushed (debounce) -> Is the max number of credits reached?
If no, send 1P output low.
If yes, do nothing.
You could use a separate hidden button wired in to the microcontroller to reset the coin counter, or you could wire some of your existing pushbuttons in series as a reset. You could also program it so holding the 1P for 10 seconds (or longer) will reset it. Actually, now that I think about it, you could make the secret reset like a secret knock, three short, three long and three short presses, or something similar.
Repeat the above for the 2P button, on a separate input/output on the microcontroller.
Next, the cost:
The Arduino will cost you about $20 (excluding shipping). The problem is that you will need a USB board or cable to program it (one-time cost) of another $20. All the software is free.
Finally, the limitations:
The coin count will have to be hard-programmed into the microcontroller. The only way to have it (easily) changeable would be to add a rotary or dip-switch to the microcontroller, and program the values ahead of time. Also, your secret hidden reset button might be easily discovered, the first time anyone sees you press it.
Want to have the coin counter reset automatically every time you switch games? If you have an existing port available on an LED controller, you could have your FE set that port on, and wire it directly to the microcontroller as a reset. If you don't have an LED controller, you will need some modifications on the design, but it can be done.
Does this all sound overly complicated for a seemingly simple task? Absolutely. But that's part of the fun, no? Let me know if you're interested, and I can help with the design in a bit more detail.