Although nobody else has chimed in yet, I'm 99.9% sure that the Mac OS ROM file is the culprit considering that several drivers are removed from it in the newer versions that don't work.
And now that I think about it some more, the floppy drive never would have been bootable -- because the Mac OS ROM file is too big to fit on a floppy!
max1zzz: if it isn't too much trouble, it would be useful if you could list the drivers when you're in 8.6 with the older Mac OS ROM that has your floppy drive enabled.
To do that: Download
MacRelix, open it up, and in the command prompt type "drvr" (without the quotes). Then hit return, and it should give you a list of drivers. It would be very useful to get the list that it spits out. I'd particularly be interested in knowing if .Sony and .MFMFloppy are in the list. Then, if you did the same thing with a newer Mac OS ROM file that doesn't have the floppy enabled we could confirm whether something's missing from that list.
I'm suspecting .MFMFloppy is going to be present with the older Mac OS ROM and missing with the newer...and making an extension that contains and loads it may be all that's needed. Maybe.