I've conversed with a guy who wore only a diaper to the beach and the cops arrested him. He got off with nothing. And when he did it again and the cops arrested him again, the judge told the cops there was no law against it and to leave the guy alone.
Personally, I can't believe he did it again, but the judge is correct. I am more covered in a diaper than I am in just a speedo! With no "exposure" and no lewd or lacivious(sp?) behavior, there is little they can do other than inconvenience and embarass the hell out of you! For this reason, when I wear only a diaper outside....I make darn sure there is NO ONE around.