a few years ago I worked in Rio flying a Brazilian registered airship under an FAA licence, the authorities don't seem to be too bothered if you are working for them.
I did notice a couple of N reg helicopters around so if you have the licence of the machine you are to fly it is not a problem.
Flying around can be difficult as they only speak English at the international airport I needed an interpretor for the regionals.
As outwest pointed out if you aren't Brazilian or Speak portuguese they can make it very uncomfortable.
