So, I ended up relocating to 21A at check-in (last row of J), as I was able to get the agent to block 21B since business class was not that full. (It was the only pair of empty seats left when I checked in.) In fact there were about 7 empty seats in the rear J cabin, and I think at least a few in the front J cabin.

It worked out pretty well -- there are some closets behind the seats, so it's quite a separation between J and Y. Service was very good, presumably helped by the relatively light load. Noticably better than the flight out on 841, on the 77W in a packed J.
I was over the wing, and thus my view was obstructed, so that was a (minor) drawback, but one that was far outweighed by having an empty seat next to me. ^