Well,
when killing fire immune mobs you have to consciously take off your shirt with a button press - whouldn't it be just as good to simply switch to Bar No. 2 where the frostbolt is on the same key as the fireball?
This might help you when there are not enough bars available for your purposes but I expect that writing all those macros that check for the shirt whould be a pretty annoying thing to do (and distributing them to 5-10 machines whouldn't be my favourite past-time either).
Or am I missing something here?
Bunny