You could do the vocal harmony on/off with a simple registration, one button for on, one for off. Very easy, but I would do that with one of the foot pedals. There is already a pair or buttons for registration +/-. So that leaves the second pedal for break. Easy stuff, even for an accordion player. Now, if you use an external harmonizer, then, your problem is solved. I have a couple of Digitech VRs for sale at a very, very good price if you're interested.

Gary