Another option is to cheat. Check this link here for a WMA to MP3 converter and then use the ID3 tag modules that are available.

Alternatively, the fellow that wrote Media Tagger seems to have cracked it. I just checked a couple of WMA's that I have. His website is here.

Good luck!

UPDATE: Check this link on SourceForge. Granted it's PHP code, but it may provide a jumping point for you.