Not sure if Taito does have one, most likely not since they moved on from the vewlix it seems. If they do release any games going forward for use in the vewlix orange/back it will probably be USB IO based.
One issue with the G1 while using the USB IO is that its pinout only shows support for P1 only, the pinout doesn't have details for a P2 hookup, which makes sense since they also use the USB control panel PCB to add controller support.
Might not be worthwhile after all to pursue making an adapter for the G1 harness.