I've realized through re-reading this thread and doing some other reading that I never understood how the original HD20 worked in the first place, that is fascinating. I've never seen or owned an HD20 that plugged into the floppy drive port.

However, re: the disk, I did come across a handmade HD20 Startup disk that I must have created long ago from a disk image I found god-knows-where; it has a 1986 version of the System + Finder plus the HD20 INIT. Is that what you meant by a disk for MacBottom, or are you thinking of one that has MacBottom-specific drivers?

This link also has more info on the HD20 Startup disk, if it helps anyone else:

http://www.vintagemacworld.com/hd20.html