Data Point: Booking made today through QRs website/app.
at no point was I able to add any FFnumber.
Called from Germany (the English line). Agent answered " sorry its not possible, you can at the airport...." (btw terrible background noise, hard to understand)
I ask about how they than would handle my free seat choice for OWE etc. He said: "please hold" , came back after a minute or so and opened my booking, ask all the relevant sec questions and then added my BA#.
Difficult and cumbersome, but now I can select my seats.
My other choice would be, to drive to my home airport since I am there tomorrow anyway and ask the QR desk during their C/I times.