The tempo reset and save again didn't seem to work unless I explicitly save the tempo.

I have hacked my way around the problem! I've been looking at the internal format of registration files at random intervals for quite a while, partly inspired by Murray Bests registration manager as that shows what is possible. All I can do is set a Song file path within an existing registration, but thankfully tempo settings are held in an easily identifiable and decodeable parameter block so I just had to reverse engineer that. Had to do some more backing to get bpm out of a midi file but at least that is documented. I have around 400 midi files that I want to organise into playlists so I needed registration bank files for all of those - and that's what I now have by wrapping this registration update code into a folder structure scanner.

Doubtless there's an easier way but it kept me quiet for a while. Probably quicker to have done it all by hand but that's so much more boring 😎