You could wear a kilt as PART of a costume just as you could any other article of clothing. For instance, if a person normally wears jeans as daily wear, he could make them part of a cowboy or biker costume easily.

Now, just because many people think that a kilt is a costume anyway, I wouldn't go as a "Scotsman" or anything like that. Instead, I would go for a historical type costume or something like Ron has done.