Please excuse the necromancing, but I felt I could add something to the discussion. This is entirely possible now, since a project called
clamz has managed to interpret the AMZ files yielded by purchases on the website to download the songs contained within the order. From a software development perspective, implementing this functionality in Subsonic could therefore involve simply wrapping the clamz tool (as already happens for the transcoding functionality), or using clamz as a reference implementation.
Is anybody interested in working on an implementation of this?