I think the key to using the kilt in a costume is make your costume either historical or go as a "Scottish __________" (fill in the blank with whatever you want. But don't use the route of the mass-produced "Scotsman" costume.

Panache's Scottish butterfly collecter is a good example. Also good would be Scottish zombie, Scottish vampire hunter, etc. Or better yet, just be a zombie who happened to be wearing a kilt.

This way, the kilt is not the costume, but is merely a piece of clothing within the costume theme.