I am not sure that getting involved in all this midi set up is going to really be a time saver--When you have a lot of technology set up, you tend to spend all your time dealing with the technology, especially if it a "first time" situation--I think it will probably be a lot more work to organize this "MIDI teaching method" than it first appears(give yourself credit--this is a whole new approach to teaching choral music!)

If you've got limited rehearsal time, best to keep it simple--if this is an existing harmony choral group, I am sure that they have some sort of method for learning new material--best to start out working with them in this way, saving the new methods until you've got time for them--

My experiences with harmony singing are somewhat limited, but I remember them vividly-I learned a few painful lessons, which I'll pass on--

First, work out your arrangement completely, so that you know exactly what you want before you present anything to the group--

Second, know all the parts inside out yourself before you try teaching them to the group--

Third, even if they don't read music, have music, with everything layed out line-by- line, measure by measure,with reference numbers for lines and measures so you everybody knows where you are, all the time--

Fourth, make notes on your score of all the changes you make, as you make them, and review it after the rehearsal, so you won't have to say "Ah, anybody remember, what were we doing here?"

Fifth, make sure everyone knows their part before you move on--uncertainty is the enemy--

Sixth, strict attention to pitch and tempo in rehearsals can avert unpleasant surprises in performance-

You probably know all of this, and I apologize for going on about it--but just in case--