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.