Greg, it strikes me that locker services like MP3Tunes are already compiling catalogs of local content. I wonder if there’s a way to bootstrap a local resolver off that metadata? If MP3Tunes has an API it would probably do the trick.
…just looked it up and found that it is promising: http://www.mp3tunes.com/api/doc/apiv1#lockerdata
About Songbird, they are now getting serious about implementing XSPF, but I think only for URLs and not for content resolution. To do content resolution I think that you’d need to write C++ against their API. That’s a pretty high barrier to entry, I agree.