>   I took a peek at the FreeBSD driver, and they have support for
> 1720K and a few other formats, floppy-tape support,
> 82-cylinder support, and non-512-byte sector support.  So that
> driver would be a good start.

Not having seen that driver, it might or might not be separated into logical
pieces the way that people would like to see the FD driver split up.  However,
it can certainly offer a source of new chunks of code, even if its structure
isn't what's desired by The Powers That Be.

Note that the 1720K format (and other high capacity formats) tend to be
write-once formats (and can't be used on all drives):  the reason those
inter-record gaps are so large is the variation in speed between drives,
as well as the time needed to find a sector and turn on the write current.
When you try to rewrite one of those sectors on the same drive, you may have
difficulty with timing slop damaging the next sector; and on another drive,
you have a substantial chance of corrupting the next sector.  However, for
what Microsoft uses the format for (distribution floppies that can be mastered
on tightly-controlled drives) it's perfectly reliable.  (I dug up this
information by doing web searches looking for a description of the format.)

And just for maximum perversity, there's one high-capacity floppy format
(1.8MB per standard diskette) which uses multiple sector sizes per track:
several 2048 sectors, one 1024, one 512 (and maybe a 256, I can't remember
offhand).  If I remember correctly, that one actually does leave enough
inter-record gap for reliable re-recording, but not all floppy controllers
allow the use of 2048 byte sectors.