Originally Posted by
layz
What I would do in this situation is put your TK number in your booking and then at BRU ask them to change the frequent flyer number to BA for that flight only.
What I would do is not associate any part of the booking with a frequent flyer number at all.
Get paper boarding passes for every sector, keep them, and retro claim.
If you need priority, or lounge access, wave your card. And mention you'd prefer not to associate your booking with the FF number because of the number of different operators.