FWIW, just ran a test search for BOS-LHR, which I'll be flying next Weds, 10/10, or six days out. I can see a number of AA metal options connecting in ORD or CLT, in both T and U. What I'm seeing is probably all married segment availability. That said, IIRC OP wasn't even able to find such married availability on TUL-LHR more than 13/14 days out, so it seems like AA is opening this availability to select markets (BOS perhaps being more "competitive" than TUL).