don't worry about the kilt as the tight woven wool will generally shed water rather than absorb it, unless it is a torrential downpour. Some tartans (3 of my Martin Mills Jura range ones) supposedly come teflon treated to resist stains, which should also repel most if not all the water. Since your kilt should be hanging vertically on your body (unless you are sitting down in the rain, or personally made in an exceptionally unusual body shape) I would think most of the rain should run off without too much difficulty.

We caught some rain last year in Scotland while I was kilted, and I never even thought about it as it was not a problem.