OK, I have it almost working. When I select "Playlists" via the top-level music selection, I see a list of playlists. The Rio did a /list/100 followed by a /tags/100. When I select a playlist, say 104, it does a /tags/104 but it complains that "The item is not a playlist." The contents of file 104 are like:

4001=TTrack 1
4002=TTrack 2
...

i.e., the tracks that comprise that playlist. I send it the binary-encoded version of that. Why does it want yet-another playlist? Presumeably the list of playlists has to eventually end up with a list of tracks.

What am I missing?