Called the AA desk to ask about it. They said they will not issue a ticket if I'm not flying "over the water" with AA METAL (codeshare not good enough -- so the CX flight doesn't count). So if I fly BA to LHR and then CX back to JFK, AA won't issue said ticket.

I'll have to fly AA to LHR. Not a big deal, but interesting to know. I specifically mentioned I will be flying on AA code and also domestically on AA to get to BOS from HKG, but they said that isn't good enough and the desk won't issue the ticket that way.