I have seen it done, but more tastefully.
The J ticket asked to be put in the back row of J and he was.
He ducked out after his meal/drinks and a friend came up and enjoyed the J seat for a while, refusing all service offered to him.
Because they were not distracting anyone by having to walk to the front of the cabin, etc, I can say that I certainly didn't care, and would have noticed if I wasn't just plain being nosy.
Originally Posted by IluvSQ
I wrote about this a little while back where 2 brothers swapped several times
during a flight. It was tolerated, but I do not think it is very proper.