KARFA

Yes so the trick using RJ MMB for AY flights booked on BA tickets should work. The problem is that the seat maps there don't show what is already occupied, so if you choose a seat already taken it just won't save it when you click to confirm. I find ideally you have RJ and expertflyer seat map open at the same time you can pick seats you know are free, and that does seem to work.

Also apparently using the MH MMB may work and may show occupied and unoccupied seats.