wasn't this done before, years ago, by Jon Lech Johansen? he wrote justeport - http://nanocr.eu/software/justeport/ (and i rewrote that in java as jjuste, but no longer have the code...)
Johansen found the public keys to allow you to stream music to an Airport Express... now we are talking about the private key, which lets you emulate an Airport Express with any hardware that is capable.
here are the keys he found - http://nanocr.eu/2004/08/11/reversing-airtunes/ and http://nanocr.eu/sw/justeport/itunesrsakeys.txt