Unfortunately the work around via RJ that used to exists and permit this no longer works. Yes, it's possible with the fare (and status).
You can try talking to BA, who may or may not be able to help depending on the agent. Then, hope that it's properly reflected in the booking...
With IB... you'd need to get an agent that can help and that won't bring up any of these excuses: 125 ticket, codeshare flight, can't see status, can't select that seat, can select but need to charge you, etc. Good luck...
It would be good to check ExpertFlyer to make sure the seats you want are actually available.