Yes, JReceiver supports multiple RIOs. I've used three at once, and I think some people have used up to 6.

The Shoutcast streaming is pretty good, but it's not pulling out the song titles and displaying them in the Rio. I think one problem with that might be that there's no way to tell the rio to "refresh" its display. And since it pulls the entire playlist at once, you can't simply update a server-side "virtual playlist" with the next song and expect the Rio to find that when it finishes the current song.

At least that's the way I understand it -- it's been months since I've had time to hack the Rio or JReceiver...

But, anyway, Shoutcast music comes through great. And someone's playing with RealAudio conversion to MP3 (which would be good, 'cause then I could get some local radio stations). Better yet would be a linux-based radio receiver that does internal mp3 streaming from an FM antenna, but I haven't even looked into that yet.