I have tried using the midi pro in Guitar Hero: Warriors of Rock for Wii and confirm that it actually does work but the game maps the notes as only the 4 drums plus base pedal, as if it was a RB1/2 drum-kit. No cymbal support.
Have yet to try older Guitar Hero games on Wii apart from Metallica, which didn't recognise it.
Now tried it on absolutely all games on wii;
Apart from GH:WoR, GH5 and Band Hero which let it work (presumably as a RB2 kit?), None of the other Guitar hero range will recognise the adaptor as a drumkit (or anything else).
Absolutely ALL Rockband games work with the Midi Pro Wii, Including Lego, track packs, specific band games...And they seem to treat it as either a RB1 or RB2 kit.
I'm very dissapointed at lack of support for metallica (and to a lesser degree the other old GH games), especially as the ps2 version works with a RB1 normal kit...(leading me to think midi-pro outputs as RB2 kit with cymbal support for RB3 pro drums)...so either I dig out the old RB1 kit and my ps2 and never sell them and the ps2 metallica game, or I buy a GH drumkit which I'd hoped I could avoid doing with the midi-pro.