I did something similar, but I was able to do mine with a standard microswitch without the lever. I still have my coin slot switches wired to '5' and '6' on my minipac, but I tapped into the p2 wiring on one of mine and epoxied a cherry switch on the side of my coin mech. Pressing the reject button in actuates the switch. I wired it as 'normally open' and when the lever lowers on the coin reject, the switch closes, registering a credit.
A small vid:
I still have to go back and wire one for the P1 credits on '5' too, because some games in MAME don't register a credit on '6.'