I've tried searching the BA and AA boards for this but haven't found an answer.

I've booked ARN-LHR-LAX-HNL through the BA site and am not able to change my seat on the BA-coded AA-metal LAX-HNL leg (I can see that I've been assigned a seat though). aa.com doesn't recognise the PNR number so I can't do it that way either - is this normal?

Do I have to call up to request a certain seat on the AA flight or can it be done online? If I have to call, would it be BA or AA?

Thanks!