I've wondered this same thing. We've been breaking 100 degrees for the past week (and will likely continue to do so). I only have 4 hens right now and their run is small (3X10) but well shaded. It is sand based and since I've been watering it down because of the heat I've noticed that it's starting to stink. I rake it out daily. I think I'm going to have to go to a different tactic. I already use the frozen water jugs and sometimes a fan, but it's so stinking hot that it seems to just blow hot air around! I've also wetted an old towel for them to hang out on in the shade but then we're back to wetting the sand. They get out for an hour or so a day to free range before it gets too hot but their predators live with us (the dogs) so it's tricky to keep them separated.
I like the mister idea and even bought one a few weeks ago but when I got it home I realized that I'd be wetting down their feed as well (tube feeder that's attached to side of run). Would pine shavings be any better? Maybe on top of the layer of sand? Or would they get moldy and keep the poop wet?