There are few caveats that I can note.
I have my new mapped "Used Genres" box and the original Genre box.
My new "Used Genre" list doesn't work with Ape, flac, MP4, WV and MP4, mkv files at first (see below). May be more but I don't have any others around to try. It does work with MP3 and Wav files.
Both boxes on the tag panel are both writable so you could end up with two Genre labels i.e. Acid\Acid Jazz.
For Mp3 and wav, Selecting Blank in both boxes will not delete them also deleting them in the main body mp3tag will not delete them, but highlighting them and selecting Extended Tags and with the popup Tags panel you can delete both from there. You can't double click them in this Tag panel and change them. You need to go back to the original Genre box and add from there.
For those that do not work, in my case Ape, flac, MP4, WV and MP4, mkv you can add via the normal tag panel or the main body. But via the main body it seems you will need to do them one at a time or via the popup tag panel as described above. You can also add them via the normal tag panel, just select them and type what ever you want in your new genre box. Then they will appear in your new Genre box's dropdown list.
All a bit fiddly but that’s what I've found so far.
I'm sure if the new genre box and the original box could be linked in some way, this could avoid these problems. Maybe when you select whichever box, that box becomes the master writable box and will overwrite what is in the other box when you save the tag, so that they match. There must be a better way, though.