There is a way - for sure. But I myself would not go the way using Mp3tag for this specific use case. Of course it's Cantello's decision how to procede.
id3.exe handles the tags - let's say - very flexible and doesn't care much about existing standards. Thats also the reason why you can write UPPER-/lowercase tags with id3.exe or as in this case a TXXX:grouping which should be GRP1 according to the ID3v2.3 and ID3v2.4 standard.
But this has already been discussed here as I read in the meantime.