For BA you can go to ba.com and select "Manage my booking" > "Retrieve my booking" Then you can click on the option at the bottom for travel agent bookings. You can enter your AA locator, last name, flight # etc. and get to the seat map.

As mentioned - BA holds a lot of seats back (EC Gold, etc.) but you should be able to see what's available. Do not do this until after you see a seat assigned on your AA res - AA will try to get a seat, if you get there first the AA effort may change your seat - it's happened to me in June.

For BA I like lower deck because you get off the plane faster. 12 J/K are great but keep in mind on BA one seat faces forward and the one next to it faces backwards. You may have someone else to walk around or walking around you. But, the seats are very good.

For IB Row 1 is the most crowded and seats tend not to be as flat (horizontal) I like the seats in the 2nd J section (row 7? - it changes a little depending on the ac type.)