Oops perhaps they do...but only for seats selected with a BAEC number? The reason I suggest it works is because of the swathes of people that use the free CX gold card that came with the Amex platinum card until last year. And then switch the number back to BAEC.

Has the batch thing been added since then? Even if or has - just keep the topbonus number in until the very last minute. BA staff at the airport can add/remove FF numbers at request. The lounge desks can do it too.

In fact AFAIK it is actually technically possible to have more than one FF number assigned to a single booking - its just that most agents don't know how to do this