I have had two round trips LHR-JNB on the 787. Both crews have said that it could not be done. I am always polite to CC. I recall one of the crew saying that - if I held the button down long enoug, it would override the lock. Nonsense.
The first of the trips was in PE and the second in UC - but no can do. I have managed to avoid the 787 on all other carriers. If it was not for the points, we would not be using it. But, as far as I could research, the rotations that we wanted out and back - were all 787.