I'm pretty sure there's also good evidence that people who are severely disabled in an accident return to their pre-accident happiness level in a relatively short period of time. I would guess it's a function of brain chemistry. Being the highly adaptable species humans are, this isn't too surprising. So sure: it's entirely possible that there were happy galley slaves and happy peasants. I'm sure there are some (perhaps many) very happy but extremely poor people living today.
The question is whether happiness has any political relevance. If one thinks of happiness in a very generic sense that can be quantified outside of social context (e.g. by the level of certain chemicals in the brain) I think the answer is very clearly no. If one thinks of happiness more qualitatively -- along the lines of 'living a good life,' or 'the ability to experience a diversity of higher pleasures' -- the answer is maybe. Although that raises the highly problematic question of who gets to define the good life.
-WD