I've written custom firmware for the uBITX 6 and got the CAT interface working well with WSJT-X 2.1.2, using the FT817 configuration. I just tried WSJT-X 2.2.2 and it no longer works.  I see in the release notes that they updated Hamlib to a newer version and one of the specific notes was to make it work with the uBITX.

I didn't have time to fully debug the interface as I was setting up for the RTTY RU contest, but I notice that it's using a few more undocumented FT817 commands. Even after adding those commands to my CAT interface, I still had trouble getting it working. I had it working fairly well as long as I didn't restart the uBITX after uploading the sketch. It seems that uploading firmware resets something in the serial interface that isn't done after a normal power up. With 2.1.2, I was able to get the uBITX to reset by setting DTR high on the interface, but it looks like 2.2.2 is using DTR differently as my uBITX resets several times instead of once with 2.1.2.

73,
Mark, N8ME