Crash, I am surprised that rain would be an issue. I have always raced motorcycles with "ram air" which would suck plenty of water in when it rained. Never any change in performance. As we know the 'Ram' effect is minimal and debatable. The scoops provide cool air which is clearly an advantage.
On my first car I wanted to see what would happen when you injected water into the carburetor. Took the hose and let the water flow in at idle. Nothing happens. I'm talking a trickle 1/8 inch thick trickle not a flood. The motor would idle along happily. You would be hard pressed to get more rain in then that.
Now I know I am opening a can of worms here as ram air and water or alcohol injection are all much discussed internet forum topics.
In the case of a race car I am just surprised to hear that you would worry about rain in the intake. Fresh air and rain would only help I would think.
John