We've tossed around the idea of an AC unit years. Its definitely going to need an alternator to power one big enough to do much in an open cab machine.
I've seen a few custom jobs with portable units rigged up. They do ok...but the cooling is very isolated and dependent on where and how you want to run the duct. Face, feet, or junk. You can't have all three with the portable units.
I'd be surprised if Polaris didn't already have a RZR NorthStar edition in the works. Who knows if it'll ever come to fruition. I'm betting that new Admiral model will have a NorthStar edition at some point too...if its ever released. And if you haven't experienced it, the Ranger NorthStar model AC and heat work surprisingly well. Same with the Defender models with heat and AC in the closed cab.