I set up a configuration for the NEC PC-6601SR (4-bit digital RGBI) and it worked really well, although I still need to figure out how to make a palette. If you already have the gear (pis and the cpld are in short supply these days) it would be my first stop for anything like this from now on...
Interesting problem. It hopefully won't take too much time to take the keyboard apart and check that ADB jack for a broken solder joint, debris in the connector, or even a cracked shield.