Like others have said, I did this earlier this year by "manage my booking" on ba.com. When you click on seat selection, it transfers you over to aa.com to do the seat selection and meal selection etc..

However, what I don't know is whether or not, to do this, you need to have the ba.com required level of membership for seat selection and, if you are blue, for instance, if this does not then work.

In that case, what I would do is just as @nalismom suggested, and get hold of the aa record locator and then use aa.com directly... Using that locator, use can use the aa app to check in as well..