The client [and probably the server too] runs the "Net-Tune" protocol which is Onkyo specific.

I haven't found much about that protocol, but it runs on Ethernet so shouldn't be too hard to deduce with a Ethernet sniffer and a client to play with.

There is even the possibility that you could make your Empeg and/or Rio Receiver work as a Net-Tune client to save having to stump up for a Client device, which is pretty clunky looking I think.

Of course the reverse of getting your Rio Central to work like a Net Tune server could be good as well.