Some interesting news to report...
I put the PCB (with InfiniKey installed) into my cab, held down start and powered it on.
Same results as last time (ie solid color screen behaving like a suicide) however I noticed something very important...
Because I'm a Jammafier user, upon boot no matter what I hold down for controls/buttons.
Controls aren't passed to the JAMMA PCB until communication with the JVS IO is established!
The handshake process, while fast isn't instant, and I think this could be the issue.
Aka from the PCBs perspective the start button is being held just after power on, but not during power-up.
I'm going to make myself a simple start button hacked JAMMA extender cable.
That will make sure soon as the PCB gets power the start button will be ON!
I'll post again once I've tested this out.
Hopefully it gets me back up and running, and if not I'll sell SBB with the key for a special (might be 100% dead) price.