I put one of these on mine:
http://www.ebay.com/itm/Blue-LCD-Di...911?pt=LH_DefaultDomain_0&hash=item2321b75f0f .
You have to slightly modify it to take power seperate from the measurement, as it's meant to monitor the voltage that's powering it (i.e. tell you the car's voltage, while being powered by the car).
You do have to be cautious about voltage drops to get accurate readings. The panel draws very little current, but if you attach the measurement pin or GND to a wire that has a lot of current going through it, your measurement will include the voltage drop along that wire.
I connected the power to the unit to +12V, which is regulated by a 7805. I hard-wired the ground to a low current ground wire (wire going to buttons) coming from the edge connector so there'd be almost no voltage drop along that wire, and my measurement pin to a 0.1" pin. I didn't see an easy way to get a - probe seperate from the GND used to power the unit.
I put a 0.1" socket on the +5V pin of the JAMMA harness, so I typically leave the measurement pin in there. That gives me a rough estimate of the voltage, though of course the actual voltage at the board is slightly lower due to the drop across the JAMMA connector. In practice, I've found this to be typically 0.01V-0.02V, which isn't usually a problem.
I left enough extra wire attached to the measurement pin that I can check the voltage at the board if I want, and also check other voltages. That model will read + and - voltages from 0V to 20V. Comparing it to my Fluke DMM, the readings were nearly perfect, though there are adjustments if you need to calibrate it.
DogP