I have a BA holiday booked for the end of October to Phuket, 3 of the 4 legs are Qatar. I couldn't select seats via BA manage my booking for the 3 Qatar legs but could for the BA leg. I managed to select seats using the BA booking reference on the Qatar site, manage my booking, as suggested by others. I can't manage the BA flight on the Qatar site so it seems the two websites don't play well together. I found with AA the BA site directs you to their site to manage seat selection, and back again if you click to leave. So BA/AA interoperable, BA/QR not - at least for me that is.