I just did this, sort of. I booked a ticket with AA, and one leg (ORD-LHR) is on BA. My whole ticket is Business Class and I am Platinum Pro /OWE, so I suspect the ticket class alone helped. This is my first BA booking. That said the second my ticket was live in the AA app, the same place it showed the AA record locator, on the BA leg, it showed a BA record locator. I am a member of BAEC, and had the app, but have never flown on BA. I went to the app, logged in, entered my record locator, and it immediately let me select a seat. It was so easy it made AA look ax antiquated as it is. I'm sure if I were in economy it would have cost; however, finding the flight was simple because AA provided the record locator.