I was flying YYZ-YVR a while back, the bird was a 773. The flight was almost full but there was an empty J seat beside me. Then one passenger with carry on bags boarded at the last minute, FA told that passenger there would be no space for the bags at the back and told that passenger to put her bags above that empty J seat. Then the FA said "you can seat here as well".
So I learned another way to op-up.