I'm studying this thread, having a smiliar kind of problem.
The coin meter in my original Donkey Kong cabinet is currently not connected. The cabinet has been converted to JAMMA and a modern PSU put in.
The coin meter uses 24V, which the PSU obviously can't provide, so my idea is to put in a second PSU which puts out 24v only.
As I would only use the meter with the original DK board, presumably I won't need the diode.
My idea is to connect the +24V to Pin 20 on the edge connector ("24v"), and also one of the coin meter wires to that pin, the negative of the power cable to a GND on the edge connector, and finally the second wire of the coin meter to pin X on the pcb ("coin meter").
Do you think this would work? I'm a bit scared wiring 24v to the pcb, but apparently that's what's supposed to happen...