I spray the runs or set up a sprinkler here on really hot days.
A few of the chickens really enjoy it, and run back and forth under the falling water. Most of them actually don't like it, and it's a quick way to get everyone to run into the coop during the day.
However, once I turn off the water and they can enjoy the nice wet grass, EVERYone loves it on a hot day. It really helps cool things off to spray the runs.
I don't think I'd go with a constant mister, but a sprinkler or something that comes on every once in a while works great here.
Good luck, whatever you do! I'm sure they will adapt either way.
trish