Good news: Interesting development, I seem to have found WMPs Media Library. Conveniently stored in xml format. A format that I invented HINI (the format I use to store settings and the Media Library) to replace. I even have code in HINI to import XML
C:\Documents and Settings\USERNAME\Local Settings\Application Data\Microsoft\Media Player\wmdbexport.xml
Bad news: Looking inside, everything of value is encrypted.
Good news: I lied, it's not encrypted. It's obfuscated. And conveniently in a format I've used before. Base64 encoding. I can read the data. Whether or not it's of any use (giving me album art data) I don't know, but at least I can use it to import files.
If I'm lucky, the data I need is stored in the WM/MediaClassPrimaryID or WM/MediaClassSecondaryID tages which do appear to be GUIDs! (The text that looks like "{D1607DBC-E323-4BE2-86A1-48A42A28441E}" in the filenames of album art or other things you may have seen) or WM/WMContentID, TrackingID, WM/WMCollectionID, WM/WMCollectionGroupID
SmallArtURL and LargeArtURL obviously point to an online resource, but the URLs are incomplete. They lack the domain name.
I also found HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Player\RecentFileList