[Banshee-List] Is there a way to identify iTunes-purchased tracks for Smart Playlist?

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I've got a couple of smart playlists for different purchasing sources 
that use an attribute of the file to work out where they're from (e.g. 
Amazon has "Amazon.com Song ID:" in the comment, while LastFM have "Free 
download from http://www.last.fm"; in the comment).

Does anyone know if there is anything similar for iTunes M4A files? Are 
there any of the ID3 tags that I can use to specifically identify which 
are iTunes purchases versus other M4As? I've not been able to find one 
yet, but hopefully there's something.

Thanks.