I can't speak for the con, but I remember seeing a report on the naked cowboy in new york and due to the laws there he wears two sets of underwear to guarantee that the shape of his junk isn't easy to see. So... in general, make it look shapely like David Bowie in labyrinth? Just like it wouldn't be considered acceptable if a woman were showing a camel toe in public even if it is technically covered by fabric.
A cup would probably be the easiest way to make sure things were acceptable and looking "good" as from what I've heard from people wearing spandex outfits.
