so since you brought it up...
I'm considering mounting a servo on my CNC lathe for indexing similar to the set-up you have here. I however am way too lazy to swap belts every time I want to change over. Have you considered putting a contactor inline with the your stepper motors and having Mach3 turn it on and off with the spindle commands? I would think once the steppers are isolated from any load even when the spindle is running you don't have to worry about any back EMFs damaging the drivers.
That's a great looking set-up you have there, and it's nice to see people hacking the tormach equipment. great job sir!