well if all they do is plug in and check to make sure that its getting a reading you might be able to MIL it and get away with it... if you turn off the rear O2's that might trip there system and fail you
my header deleted my cat, and I fixed that with a set of anitfoulers , it will probably work for the mustang you just need one on each side,
buy a set of 1/2in anitfoulers from autozone (help partnumber 40002) leave one alone, the other drill out with a 1/2in drill bit, screw the modded anitfouler into the normal one and screw your o2 sensor into the modded one
o2 sensor--modded anitfouler--regular antifouler--exhaust ( do this on both sides of the exhuast for both o2 sensors)
and you have no CEL cause it limits the exhaust the o2 sensor sees so it thinks you still have the cats on.