i dont see a problem with it, but...
If you wear your kilt how you wear it every day, you know, with a t-shirt and your hose pushed down I dont see that as a costume. However, if youre wearing your kilt with a jacobite shirt, and a swordsman vest or something like that, or maybe a great kilt then i would say that would make a pretty nice costume. Most people dont see kilts as daily wear, so they think of them as "costumism" I think for the most part anyways.

I ordered a Gryffindor house sweater and tie for Halloween this year. lol Im a Harry Potter dork though, so I imagine I'll wear them after Halloween as everyday clothes too. I think the crimson and gold should match my kilt pretty well too :-)