Well, there's a couple problems. The first is that playing the drums, piano, or guitar is heavily based on finger dexterity, which can be simulated easily with buttons. Brass instruments feature a combination of finger dexterity, lip control, and breath control, which is much more difficult to simulate using a video game. Your peripheral details one of those breathing meters, but I can't remember what they are called. However, there are problems with this system: The first is that simply breathing into a mouthpiece itself is by no means similar to buzzing into an intsrument. The second is that the mouthpiece of this could not be shared with players, so the game would flop as the group activity Guitar Hero is.
Real Instrument play would be tricky. I own a silent brass mute for my Euphonium, but I can tell you, the one I own is terrible. The microphone pick-up sounds worse than just using a mute at all. If all mutes caused affected the tone of the instrument in the exact same way, this would be fine, but I find that different brands of microphoned mutes produce different effects, unfortunately.
Conductor mode would be great. I'd like that, and I'd prefer if there were an authentic mode and a beginner's mode. If there were any way I could actually control a band or orchestra with the Wii-mote, I'd enjoy it. Controlling tempo, not so much. Also, Flapping conducting is not my style. I hate how they looked on stage, it was more like they were tying to fly than conduct. I'd want a whole lot more accuracy and realism than that.
I hate to say it, but I don't think this game would work commercially or conceptually. I hope someone proves me wrong, though.