There is no regulation preventing rest in any seat on the aircraft. In fact on aircraft without bulk crew rest we have 1 j/k kept for us just for that purpose. Anything to the contrary is rumour and nothing more.
That said, I would never inconvenience a customer whilst getting [U]required[U] rest. If a seat had been allocated which potentially could inconvenience a customer, I always went and spoke to them to explain the situation. I never had a problem. Sadly, we do have some idiots that feel the world revolves around them and do not display simple common decency. If a seat has not been allocated, discretion is the word of the day.
The vitriol expressed to Manuel is sad, he has a point. That said most people that eat in swanky restaurants are always trying to get on the good side of the maitre 'd so that they get a table in the first place.