Originally Posted by
1Aturnleft
The only way to get around seating is to input your AA EXP into your BA booking. If you do this you'd need to update your booking with your BA details at check in or at the lounge and get a boarding pass reprint with your BA number on it. Do not scan any boarding pass at the gate that shows your AA number on it as it's at that point your booking will track/credit to your AA EXP and not to your BA.
this, and yes to the earlier question about flashing your card to enter lounges or check in areas. that is exactly what i had to do for the year i switched from AA to BA. requires a bit of a think the first few times to ensure you do not lose out on the BA tier points but you get used to it.