I don't have time to work on coding this for at least 2 weeks so I thought I'd throw it out. Does this seem like a valid approach, and is there anyone far enough along to work on this? If it's of interest but nobody gets to it, I can work on it when my schedule gets clearer.

I tried this myself. I set up a multicast group and then when the Rio was about to send the audio to the sound card it would just write that same data out on a multicast address. Unfortunately this caused gaps in playback on both the master and slave Rios. I'm not sure if the limiting factor was the network or the CPU. Plus, synchronization is still a tricky problem. If anyone has any ideas on how to solve this, I'd like to hear them.

Unless my math is wrong, 48000 samples/second * 20 bits/sample * 2 channels (I assume these are stereo streams) * 64 streams = 122Mbps. If they're actually capable of this, they must be compressing somehow. Even if they're mono streams, that's still 61Mbps which in my experience might be possible but definitely at the edge of what 100Mbps Ethernet is capable of. Certainly looks like an interesting technology though.