Those actually seeking a way to resolve seating issues on BA/AA mixed bookings (rather than simply posturing about how awful the BA/AA integration is) may find some answers on this thread
http://www.flyertalk.com/forum/showt...ferrerid=26955.
It is pretty clear how to resolve seating issues for AA metal segments on BA bookings and several methods are discussed in the referenced thread (which is only 2 days old).
It is slightly less clear how to resolve seating issues with BA metal segments on AA bookings but I would have thought the reverse lookup of the BA PNR using Checkmytrip and then using that PNR to adjust seating in MMB may be one approach. I do not have any AA originated bookings with BA metal segments to test.