I'm in exactly the same position unfortunately flying y (Qatar LHR BKK) I tried seeing if it would let me choose a seat for free well it didn't but I selected the seat's and abandoned the process just before the payment page however it seems to have kept the desired seats as reserved and the seat numbers are visible on both BA & QR websites

Incidentally the seats are visible on expert flyer as occupied

Why did I book a QR flight through BA you might ask unfortunately dreaded vouchers so it booked me into N class no other choice than full economy in BHY which was crazy money