I did a little research on this topic, because I am considering something similar for my own music streaming needs. This would be a very very cool setup, indeed.
It will be quite an undertaking. You need a DACP server to handle the conversation with Remote. To the best of my understanding, there is not currently an open DACP server, although there should be enough information out there to write one. There is an open client [1], and the pairing protocol between client and server has been broken [2].
Then you need an RAOP [3] server to stream the music to the Airport Express. Fortunately, there is a free RAOP player out there, raop-play [4].
Architecturally, these components should not be included in a DAAP server (like Firefly). Instead, they should be separate so that the DACP controller / RAOP player could communicate with ANY DAAP server, including an iTunes machine, via DAAP. That said, it would be nifty to have a package which included all these components in one handy place for users.
[1] DACP client: http://dacp.jsharkey.org/
[2] DACP pairing: http://jinxidoru.blogspot.com/2009/06/itunes-remote-pairing-code.html
[3] RAOP: http://en.wikipedia.org/wiki/RAOP
[4] RAOP player: http://raop-play.sourceforge.net/