Using an airline's own website, you will often find these odd connections when a codeshare flight in the middle would make life a lot easier. The CO
website does it pretty frequently.
Your best bet is to go to aa.com, and click on more options. You can select "American Airlines, Eagle, Connection & One World" from your airline choice. That will bring up BA
flights and you would then just book the entire ticket through AA
's website. You won't be able to pay for it right away, but I think you could make the reservation.