Bridges all available devices so once instance is enough for the entire network.

AirPlay->UPnP

NodeJS module, using node-avahi to handle incoming connections and bridges them to UPnP calls, probably using node-gir to bind GUPnP.

UPnP->AirPlay

Phase 2.

Ideas

Use a mDNS service announcement to ensure that only one instance of this is actually active on the network.