Everybody I know who actually flies down to Africa regularly nowadays (probably nobody who has posted so far) says that a pilot uniform works wonders.
Not that I would suggest getting a white nylon shirt from a N European pilot shop; you will smell worse than a pig in the heat down there.
It is nothing to do with impersonating an ATP. It's just that in those countries the people are used to blindly obeying authority, and uniform=authority. So it lubricates the process, so to speak.