It is possible, if you manage to find an old MFM drive on a scrapheap somewhere. I managed to get a 80 MB seagate MFM drive working in a megafile 60 once, and that worked pretty well. If you can find some OLD XT's somewhere, you might rip out the harddisk and give it a try. I am not sure if much bigger MFM drives were ever made then 100 MB anyway.
Maybe time for a scsi adapter ?
(or a big stack of old harddisks to shuffle around.. ?)
RE: upgrading megafile30 HD