It's interesting because apparently BA will interline on the same PNR and AA will interline to other OW carriers if on the same PNR, even if technically separate tickets (both airlines currently have no-interlining policies, even to themselves, on separate tickets).

There is some discussion in https://www.flyertalk.com/forum/brit...e-tickets.html (BA forum) suggesting that BA may work this way because it's just how the system works at check-in, so I suppose it could still be down to how the airline pragmatically handles this situation rather than any overarching policy.