Hi mjwaldne,
While helping a friend with his mill recently, we actually commented out the line for the "-1" selection (no axis selected) and using a 5 position rotary switch made the first position (most left or CCW) the off position, isolating the A & B phases. Had a few spare poles on the switch to do it. Then X, Y, Z and A for the other four positions.
On my mill, I will also comment that line out and isolate the Phases, just seems a safer way. I found if the MPGs can stay active, all is fine while running G Code as they are rendered unusable, as are the on screen jogging. But when you hit feed hold, then if you somehow accidentally touch an MPG wheel, it starts the G Code running again. I am sure there is a way of stopping this somehow in the code. But I am not there yet. So hence the want to have a hard wired off state somewhere. Guess what I am saying is, I am not sure that line in the code is good enough. Doing it in the hardware is the best.
Craig