Undoubtedly there are other materials that can make a buckle other than metal, but for getting through airport security it is not an issue. I've flown kilted often, as have others here. If the detector goes off, a quick pat over the buckles by the TSA rep to ensure nothing else under there, and happily on my way.

Your other choice might be to have the kilt made, and then replace the buckle with plastic hardware used on backpacks and such. These are easily found in fabric shops and camping supply stores.