Paid for seating is an option for tickets not issued on ba 125 stock but the ba metal flight must have been booked under the prime ba flight number rather than the equivalent aa flight number. Unfortunately this means that you will not be able to pre pay seating on this booking.
Do not be overly concerned about the message suggesting e tickets have not been issued as they do appear in the ticketing pnr (aa) but will not necessarily transfer over onto the operating pnr (ba).