While it is possible to use the Psion as a data transport; you're right, the serial transfer is much slower than the empeg's USB connection. Also, Psion memory is usually limited or expensive (or both.)
I was thinking more like custom playlists that one could create on the Psion (based on the MP3 data already in the player) or as a keyboard to enter or correct the data fields in the empeg, etc. Since the actual playlist data is stored in the empeg and not back at home in emplode, it should be possible to modify. How about a keyword search using the Psion (or even just the Psion's keyboard... allowing the empeg to do the crunching!) Lots of possibilities, as both devices are very flexible.