In an attempt to collect more cash from travellers, it appears that AA has removed the ability for passengers on BA tickets to select seats in advance. If you call AA they extract a $35 fee because the ticket was not purchased through them. If you are BA Gold/Silver you should get away with the fee. The BA web site links you to the AA web site for these segments. The AA web site then takes you back to the BA web site and at no point can you pick a seat.
