OK, sounds like it's a totally different problem then, even though the symptoms are similar. I looked deeper into my problem, and it seems that the CPU is stuck forever at priority level 1, presumably in some interrupt handler that never completes, so the next VBL interrupt never gets processed. I don't have enough diagnostic info to tell exactly what it's doing, unfortunately.

That's interesting about the Sony driver function pointers. I don't believe the Mac System software patches the Sony ROM routines at all. I could be wrong, but if it did patch them, then anything that used a custom ROM to replace the drive (like Mini vMac or your ROM disk) wouldn't work.