MP3Tunes search interface: http://www.mp3tunes.com/api/doc/apiv1#lockersearch
Search results include playable URLs:
http://www.mp3tunes.com/api/doc/apiv1#lockerdataexampleresultalbumtracks
Results are available in JSON:
http://www.mp3tunes.com/api/doc/apiv1#responseformat
Combine that with JSPF:
http://jchris.mfdz.com/static/jspf_parser/xspf_parser_runner.html
And you get a local content resolver in pure AJAX using odds and ends that are already available:
1) Get an XSPF file with metadata fields filled in
2) Parse it using JSPF
3) For each track with metadata, call mp3tunes search API
4) For each search result with a playable URL, fill in the corresponding track location in the parsed XSPF
5) Activate an in-page player which can read an auto-generated playlist