No, they have not - it is not nessesary because it is never too warm. Average temperatures are up to 25 degrees celcius.
They do not have air conditioning, and the fan provided was pretty anemic. The only time this presented a slight problem was -- as I think I said in my review-- in the afternoons on the warmer days, because the our room had a west-facing window and porch. (We were there in early April.)
Get an answer from Mana Nui Inn staff and past guests.