If Brian said it, it's likely to be true. I worked for him in the early 90's, and his post-AA gig sounds way cool.
BA is still the only operator at T5. Oneworld is still fairly consolidated at T3 on the routes served by at least two of the carriers. All the BA metal to SYD/MEL/PER/BKK/SIN, BCN/MAD, HEL, LIS, and NCE is operated out of T3, and they just upgraded the lounge pavilion from what I recall. Main reason -- with the exception of NCE and LIS, these markets are heavily shared between BA and QF, IB, and AY.
They also codeshare with CX on LHR-HKG, so I'm a little surprised they don't run HKG out of T3.
At JFK, remember that BA also has a long term lease on their terminal, so they're not in any hurry to move unless the Port makes it worth their while. Also pretty certain that BA's employees at JFK are unionized (likewise for AA's in LHR), so there won't be any "contract handling" if they do co-locate. You'll simply see AA sub-lease out dedicated space to BA, and they'll handle their own flights. TWU can piss and moan about space being rented out, but last time I checked it's not a scope violation to sublease real-estate not owned by AA.