But if by "qualifying for BAEC tier level" the OP is referring to the "two BA flights" or "four BA flights" that are needed to qualify for a tier in addition to the TPs, then AA code on AA metal won't count.
I don't know about the veracity of this statement. But I'm pretty sure that the OP can easily book a short-haul return for a fraction of the 300 pound price difference. It's not an ideal solution, of course.