Right, I’ve discovered my first bug, If you have properly tagged m4a files, my script won’t submit musicbrainz track id tags as specified on the tag mapping page.
I actually think this would be the best bet as an “alternate” for id3v2 as well as it matches the existing naming convention of all the other MBID specific TXXX frames as well. However, if you guys want to list others, I may add them.
edit: An updated version with a fix for the m4a problem has been uploaded. I’ve also updated the first post mentioning that musicbrainz_trackid or musicbrainz track id written as TXXX may be used for files containing id3v2 tags. This is because foobar does not support and isn’t likely to support UFID tags.