I wondered about this too, having spent many years in various choirs, you anticipate the motions so you are ready when the baton hits the point that you know it will when it is time, the acceleroneter giving continuous rumbles with variation whenever it changes course (the upbeat and down and side to side would likely get the job done. It would take some practice but likely would be workable especially since each piece would have been rehearsed many times beforehand. The nuances would be more difficult to accomplish, much of the dynamics have to do with reading the eyes, the "free" hand, the facial expressions, is the baton pointing at my section or another, ? This is likely something that would beg for more but i imagine a blind man is used to picking up on other inputs far more than we might imagine they could, and again considering that they practice many times, it would seem it could move them forward quite a ways. Best of all, it's simple. The singer could learn to anticipate the proper interval to be there on time even with a bit of delay in the siganl feedback as long as the delay is always consistent.