I have an upcoming AA --> CX and return, and I was more or less banking on this policy for the outbound, not sure how it would work on the return. It sounds like the "checking an empty cardboard box" is a fantastic idea, since I plan to not have any luggage. It would force them to "link" the tickets at the airport, whether or not they could issue a boarding pass (although you would think if they could check the bag, they could do the BP too. But you never know.)

Thank you OP for getting something more definitive!