After a bunch of research and trial and error, I finally got the coin-in functionality to work.
This type of IGT machine uses an ABC coin optic detector translator. Basically what happens is the coin is dropped into the coin slot and passes through 3 different optics (A, B & C). As the coin passes through each optic, the optic's state changes from zero to one for a short period of time. This sequence is important to the game program and must be in the following order:
So first the A Optic is triggered, then the B Optic is triggered (while A is still active) and finally the C Optic is triggered (while A & B still active). From there the optics begin to shut off starting with A, then B, then C.
The reason for such complexity is to prevent "coin stringing". Where someone attaches a coin to a string and tries to pull it back out of the slot. In this scenario, the optics would be triggered correctly on the way down, but would start to trigger again on the way up and screw up the sequence expected. Once the sequence is broken, the coin either produces a "Jam" error message or "Coin Sequence" error.
By adding code to the driver to simulate a correct sequence of the optics, I am now able to press a key to insert coins, and the game shows a coin in.