Just a few thoughts for what they're worth

Why not use a contrasting patch sewn onto the kilt such as tartan or a different colour canvas and attach the pin to that?

Failing that, there are some kilt pins with rings on the back designed so that the pin can be sewn on. Wallis-Hunter do this with most of their kilt pins but they are solid silver and fairly expensive. There may be others around though

A final thought is simply to close the clasp on the pin and sew a few threads under the face but around the pin itself and going through the canvas. A few stitches at the absolute top and absolute bottom should do the trick