Is the ticket issued by AA (001 stock)? Do you have an AA record locator? If yes, there is a trick: try retrieving the PNR using it on Qantas website. There is usually a "hidden" (also known as "operational") BA PNR for such segment, and Qantas reservation management system will disclose it for you. If it does, you can add it to MMB on BA website or app, add your BAEC number to it, and choose the seats. That worked for me in a similar situation, I had BA3 booked under AA flight numbers.

If it doesn't, you should not be disappointed as there are no bad seats on LCYCW flights .