I thought the ultimarc j-pac was made for that purpose?
it allows a jamma set-up to display on a pc monitor.
A Jpac allows you to interface your existing control wiring to the pc. It also allows has a video amp, and a chip that blocks any signal other than 15khz and/or cuts it in half to diplay.
It still takes a bunch of work to get your pc to output a valid arcade monitor signal.