I'm not aware of a non-Mg rule for wheels or anything other than what's in the cockpit. They don't want any in the cockpit area for sure, in case of fire. Never had a problem in Tech with my Mg front wheels even when I'd mention them, but then again before I got it the car ran SS for many years with an aluminum driveshaft loop, among other "extra-legal" things...
A Mg fire is notoriously hard to put out, however, so it wouldn't surprise me if some inspectors frown on its use on non-vintage cars.