Colin,

At first I thought a sweater under a "suit style coat" would be too casual plus the fact that I've never been too crazy about sweaters under coats.

After chewing on it a bit, I started to visualize that the sweater vest gives the same visual context as a waistcoat would.

I'd say go for it. But you may want to use a neck tie that incorporates a color from the kilt to tie the top and bottom together visually.

Not an educated opinion, just what I feel right now. Have a great time!