How do you want to play it online? You would need teleportation to get to the correct station!
You'd need a server to make that happen. Also on each CPS2 you'd need an adapter to com port (IIRC that was RS232) and a client running on the PC. Not that it would be very complicated on the software side, but rather on the hardware. I guess you could also do it just with an arduino on each client...
if you can develop an adruino board or an adapter that hook up to existing arcade hardware CPS3, CPS2 and connect to a server and allow people to play online, I will be down for to buy some. I've been dreaming to play Third Strike online with friends on actual CPS3 board. Please make this happen