I do not think that it would be illegal for a female to play the role of Boy George in a movie. Many other females have done so in the past. For example, in the movie Peter Pan, several females played the part of Peter including Nina Boucicault, Betty Bronson, and Sandy Duncan. There were other reasons why these females played the role of Peter (lightweight for easy lifting) but they were very successful at playing the role of a male nonetheless.
Boy George had a lot of feminine qualities so a female playing his role may be a good thing. It may be very difficult for a male to do so.