Just a quick update, the solution above works, but you need to connect the Echo to a Windows PC running TuneBlade via a physical cable (the speaker output from the Echo):
Exho —(cable)—> PC —(stream over IP)—> AirPlay Device(s)
You can tell the device to which you are streaming to, but once you conncetect that cable, you can’t really hear the Echo other than through the PC or Airplay. Not exactly the solution I’m looking for.