I assume you'll be using RGB (SCART) out to hook up to a real arcade monitor?
I'm doing something similar. I've bought an LM1881N chip, and a SCART female socket. I'm getting all the signals out that I need (R, G, B, Comp Sync) and feeding them into a DB9 switchbox (use DB25 if you need more inputs, or whatever). My switchbox will then select between the SCART socket for consoles, and JAMMA input for arcade boards.
You could either remove the switchbox from the picture and just use the SCART socket, or remove the scart socket and permanently attach the consoles to the switchbox and simply switch between them.
Further to what I'm building, I'm getting a whole bunch of JAMMA fingerboards (jammaboards.com have them with additional "kick harness" attachments for 6 (or more) button setups) and attaching them to some gamepad hacks. That way I'll have a generic series of JAMMA->Console controller hacks, and a generic SCART->JAMMA converter, making swapping and changing of my consoles in and out of my arcade machines nice and easy.
Again, you can build just a small subset of all of that and have what you want.