Rust? Do you guys have cast iron recievers or something?
Like Hector said, don't worry about it. When CAG was together, we used to go to BT and play in the rain all the time. I remember one day it was raining particularly hard, and we simply wrapped our gats in a garbage bag. CAG D Boy, being the youngest, didn't get a bag and his AEG got really wet. At the end of the day he dried the contacts and was GTG.
Not to prove you wrong Andy, but I think a light coat of oil would be better suited to firearms than airsoft guns. After all, there is no exposed action/bolt like on a firearm and there are no exposed metals that will oxidize with a small bit of moisture. A bit of corrosion might occur at the electrical contacts, but they are within the rifle, and will likely remain dry throughout the day.
If you want to play on the safe side, put some duct tape around your battery connectors, and try not to dive into any puddles. Anything more than that is overkill! Just remember to dry it off at the end of the day.