My bike currently has the right shifter integrated into the brake lever using the I-Spec mount from the XTR 10-speed group set, like the first picture below.
If I upgraded my drive train to XTR 12-speed but not my brakes, could I just screw the aluminum I-Spec adapter from the old shifter into the new 12-speed right side lever, replacing the band strap as a hack I-Spec mount?
(I wish there was a way to reduce the size of these images!)