What specific model WG monitor are you using?
Unless you have a mutlisync monitor I think your stuck.
You would more than likely have to use something like this:
http://www.ultracade.com/hardware/18/uvctradeRun it straight to the monitor.
I don't believe you will even have to run it through the JPAC at all.
Think of it like an exrenal Arcadevga graphics card.
And if you went this route, I don't think you would need the Arcadevga card anymore for the desktop pc setup either.
I don't personally mess with much of the MAMEing type gadgets, so maybe one of these other fellas can explain it better.