I love the pin, but I'm wondering if it'd get swallowed up in the fabric. One of the nice things about a round brooch is that it holds everything flat. But I think that ancient style is exquisite, so try it!

And yes, you'll have to pin it through the jacket. There's a lot of weight in a plaid, and it'll come right off your shoulder if it isn't pinned down. The pin will go through the weave of the fabric, so it shouldn't leave any holes. My brooch is a penannular I forged, with a forged pin, so it's on the thick side, but it leaves no holes in either plaid nor jacket.

Nick