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It's far enough along that it can list the whole library, and a client like Rhythmbox can request and successfully play a song.
I stopped mostly from time constraints, but I'm hoping to have time to continue working on it. Things like seeking in tracks and playlists would be nice to have.
I've started this: https://github.com/dpogue/gmusic-daap
Unfortunately the DAAP library I started with doesn't support some of the options that iTunes looks for. It works with Rhythmbox and Banshee on Linux, and someone told me that it worked with an Android DAAP client.
Unlike the API posted here, which emulates requests from the web interface, mine is based on the internal API used by Google's Android client.