Originally Posted by flatlander

With an AAdvantage number, I usually have to enter it during the booking process on BA.com which does not let me choose seats, then immediately after I have finished and paid for the reservation, go back to BA.com/mmb to manage my booking and choose my seat. This rarely fails, and if it does, you can call BA.

The seat choice immediately after booking is very nearly the same as that during booking, the book rate or seat change rate on a particular flight isn't high enough to be material during the few minutes between starting a reservation and finishing it (when it's automatically ticketed on BA.com).

I don't think I've ever failed to get the seat I had my eye on during the booking process when I went back after booking to MMB to choose it.