how bout a water dispersion system in the top of the helmet. You can have channels that lead outward from a hole in the top so on hot days you can take your water bottle and squirt water in the top and have the water evenly distributed across your steamy mellon.

Make some channels to help keep it out of your eyes, and maybe something to let some drip down your back. You could even have some way to block certain passages say if you don't want water down your back or the front ones put water in your eyes.