This issue crops up from time to time, but the fact that a tartan may be copyrighted doesn't mean you need to seek permission to wear it. It means you need to have permission to produce it.

Think of it this way -- songs are copyrighted material. But you don't need the artist's permission to listen to it. On the other hand, you would need their permission to record, produce, and sell it.

The same thing is going on here. Once you've purchased the cloth, you shouldn't need anyone's "permission" to wear it as a kilt.
Aye,
Matt