Many thanks to AJ (Convergent MightyFrame), I recently acquired an actual UNIX PC! along with one of David Gesswein's super cool MFM emulators. I've since added a gotek running the FlashFloppy firmware which went pretty smoothly. One minor problem, I had no mouse. After some experimentation, I was able to get a PS/2 mouse wired through an Arduino for use on the UNIX PC. The tricky part was realizing the serial comm is inverted. Ideally I'd like to make a small Arduino Nano shield for a more permanent solution, but I need to learn KiCad first to design a PCB. Anyway, for anyone that's interested, I posted the prototype here:
In the future, I'd like to try emulating a keyboard (or using a ps/2 keyboard), but I didn't have much luck injecting key strokes into the mouse port -- I think this needs to be injected directly into the port on the UNIX PC which I assume goes straight to the 6850 ACIA. I'm guessing there's a chip in the keyboard that generates the protocol that goes to the ACIA. e.g., this protocol has a keyboard_begin character and a mouse_begin character depending on what's being sent.