Not sure this helps but is a way to find out the AA record locator from the BA reference number, that way you can use the AA website to choose seats on the AA flights.

Take your BA flight reference and use checkmytrip.com to find the AA record locator.

Enter the BA reference and surname in the top of the checkmytrip page.

Expand the details on the flight information listed and you should see:

Confirmation Number: American Airlines xxxxx

Use that reference on the 'Manage My Trip/Online Check in' section from the AA.com homepage.