Does the reservation reflect your AAdvantage or BAEC frequent flier number? if BA, this will be the case. if you have your AAdvantage number, I believe that you should be able to make some level of selection as PLT (I know that we can as EXP when the AAdvantage number is there).
If it is the BA umber, you can't change it on BA or AA site, but you can change it on Finnair.com or RJ.com (Royal Jordanian) sites using the BA PNR.
Cheers.
I have not yet booked. But it will reflect my AA platinum number. But ticket stock will be BA, and metal will be AA. Can I still chose seats on my AA flight? Had I booked directly via AA I'd be able to chose seats free of charge - and request 500 mile upgrades on domestic segment.