Any other year, I'd say "after our arrival" (on December 28) would not be possible. My carioca friends usually plan a bit further ahead than that, despite the last minute-ness of Brazilians. Maybe this year, with the extreme financial crisis in Brazil and relative lack of tourists, it might be a bit different, but not sure I myself would count on finding a good place at that point. I suppose you might find out about what it's like to be on the beach all night.