My understanding is that playlist 100 is a "master playlist" and contains references to all other playlists

Yup.

I've seen request from the Rio (running the stock client) for "/tags/100". Does that mean it wants ALL the tags for ALL tracks?

No, just the tags for the "master playlist" (of which it probably uses just the name and the length).

I've also seen "/list/100". Does that mean the Rio wants ALL track ID info for ALL tracks?

No, just for the direct children of the master playlist, i.e. all the other playlists (not tracks).

Peter