The rio takes 12v; problem with POE is I don't think it could power the audio amp in the receiver (or not at full volume, anyway). You can solve this problem by just pulling the amp board out though....

The rest of the board doesn't actually need 12v - 7v is probably ok. Almost everything runs from a 3.3v switchmode PSU, apart from some analogue that runs from a 5v linear regulator (for cleanliness) and the core voltage which is another LDO.

Current is minimal unless you're running the PNA chips, which I guess you wouldn't be in this application - definitely less than 300mA.

I'd say it was definitely worth trying if you can get a POE chipset eval board... ISTR either Maxim or Linear Tech do some almost all-in-one solutions.

Hugo