I'd expect that by now anyone who actually already knew how to access the MUs as raw device files would have piped up, I've been talking about doing this for a while. Someone might be able to work it out, though, if they went looking.
If someone sufficiently technically inclined does happen to be reading, go look in the xboxhdm source code and see how it opens the hard disk as a raw device in order to get access to sector 0 of the internal HD (which is outside any partition and thus not accessible via the normal filesystem access calls). All I need is the required device name to do that for a memory unit, which I'm moderately sure is possible (it's just USB Mass Storage after all and the NT object tree should include an entry for it) - it just doesn't seem to be under any of the names I've guessed
If I get the time I might implement FAT anyway using disk image files; not useful to users as it won't be able to use a real disk, but it would then be a simple matter for someone else to switch out the disk image for the Mystery Device needed to use a real MU. But.. that's less cool and exciting than doing it properly, and Xbox hacking is hardly my only project. I have other things that are more immediately fun (and the attention span of a goldfish).