Some progress: I actually got it working on the Lisa under Macworks, for both reading and writing. But for some reason it's super slow. I think it must be encountering tons of low-level errors, but it keeps trying, and eventually it succeeds. I'll keep at it.

What I found is that the method I'd been using to generate the special 10-bit bitslip markers was broken. This is one of those things where you wonder how it ever worked, and it must have been causing problems on the Macintosh too. I played around with it a little until I got something that worked (slowly) on the Lisa. Now I need to understand why it's so slow. It will also be interesting to see how this change affects floppy emulation on the Mac - maybe it will be a little faster, if there were resync errors happening before that I wasn't noticing.