I've thought about this for a while (although when I needed to use the empeg as a file taxi, I was able to dump a zip down via emplode then use the serial port to recover it), mainly because I have a spare MK1 sitting around the place =)

(Disclaimer: I haven't looked into the technical issues in any detail, so I might be spouting rubbish... =)

The emeg USB driver has to support 2 way transfers (it's how the playlists are retrieved after all), and we have the source to emptool, so we should be able to figure out how to push files backwards and forwards using the player software on the empeg talking to a PC program via the USB driver (If we can work out how to talk to the PC USB driver, I assume it won't have a com/corba interface?)

However, it's possible that the USB from the empeg to the PC would be slow (as it's unlikely to be optimised for large files going back to the PC).
It's also very likely that the player wouldn't be able to write the files into a "Taxi" subdirectory, so the they would be mixed up with the music, and there is the risk of it appearing in a playlist (although it won't actually play it) or being overritten.
Finally, if the Sync protocol changes, the Taxi program would need modification.

Just some random thoughts...
I'll have a poke at the USB driver at some point to see if it's willing to talk to me... =)

Jazz
(List 112, Mk2 12 gig #40. Mk1 4 gig #30. Mk3 1.6 16v)