It was kind of a pain, but not too bad. The GCR stuff is almost the same as on the Apple II, which is documented in books like Beneath Apple DOS. You can find out the Mac-specific differences by looking at the MESS emulator source code, or just disassembling and reversing portions of the Sony driver from ROM. Info about the sixteen 1-bit registers in the drive is partly covered by inside Macintosh, and the rest again comes from emulators and ROM analysis. The IWM itself is not that complicated, and is pretty-well documented.

Also I think it was easier to create a replacement for the IWM and floppy than it would be to create a replacement floppy that works with a real IWM, because replacing them both means you're mostly freed from worrying about disk read/write timing concerns.

I'm happy to share what I've learned with anyone who's interested, but it's really nothing new, more just a compilation of info from a bunch of scattered sources. And as of today, my function names and comments for the Sony driver in the Mac Plus ROM have been integrated into Paul Pratt's FDisasm fdplus-0.4.0.zip, if you want to check them out.

Anyway, no one should get too excited until I can actually boot this thing into the Finder!