I have just booked a multi sector BA trip with an AA JFK-SFO in the middle of it.
Seperate RL for AA showed on my TA confirmation.
BA.com shows no seats for AA and no changeable. Had to go to AA website and change using their engine but even when allocated the seat selection did not show up on BA.com
FYI, even though I am Emerald I cannot select prefered seating and you have to call AA to get access to the "purple" seating on the seating chart which is a PITA.