You can absolutely do cycle-accurate full system emulation in MAME now, but you need to start from a cycle-by-cycle capable CPU core (one that can run only one cycle of an instruction rather than always running at least 1 instruction).
Yeah, I did set up callbacks for the 8X305 core that let me perform operations on the board at the quarter-cycle resolution that it needs. Hm, okay, I'm feeling like I can get back into this. At the very least, I can push up what I have so far (cycle-accuracy for the 8X305, more VMEbus control lines, and enough of the '320 for it to say it can't find sectors) and go from there. I started taking notes on the timing circuits and dividers as well.