Are you wanting to use a Raspberry Pi to store all of your music? Or do you just want each Raspberry Pi multimedia center to be a target to play music to?
If it is #2, then you should be able to install shairport on each raspberry pi to make them Airplay targets. Then Plethra should see them as a location to play to. Chances are they won't be in sync with other computers and airport expresses, but they should at least play in sync with each other.