Been too busy recently to do much with this but had a couple of mammoth wirewrapping and code hacking sessions this last week.
What I've done is add a Vinculum VDIP2 module. This makes interfacing with USB storage devices (and other USB devices) a doddle, it's got its own firmware that handles all the FAT32 misery and just presents you with a few simple APIs to open, seek, read, write bytes to and from files etc.
I was given the inspiration by RetroClinic's DataCentre interface for the BBC Micro. This lets you fill up a USB stick with all your favourite games as disk images and use them from the BBC just like normal disks.

The biggest challenge has been to port CP/M. There's not a lot of room in the BIOS area to fit in all the stuff I'd like so I think I may have to cheat and move some of the code for accessing the Vinculum into ROM to free up a bit more RAM and allow for a little image file selection menu.
Anyway here's a couple screen shots of CP/M 2.2 running (via a serial link). It's read-only so far, though to save me constanly corrupting my usb sticks when I make a mistake!

