hmmmm, so this buffer is for the "raw" audio?

a couple of ideas that might work...
1) Starve the buffer (so it always needs a frame from the wire to play)
2) Broadcast the metronome (current frame) again a flat network, everybody should get this at the same time
3) Bypass the buffer (is this possible?)

It's been a while since I looked at the kernel code, but if I remember correctly, the audio device uses DMA to read out of this kernel buffer. Because of this, I don't think starving the buffer is much of an option, same for bypassing the buffer. As for the "metronome" as you call it, this is basically the same method that I've been trying (broadcasting a frame number to play). This still has the issue of making sure that each Rio has roughly the same amount of data sitting in the above mentioned kernel buffer.

So is the rio a 10mb or 100mb device on it's ethernet side?

10mb.