You know, clothing companys have been making a product similar to this for years. It's called a pocket. Generally they're found on the sides of your jacket and more often than not the sides of your pants too!

I can't see spending extra money on a mitt that fits over my release when I can just stick the whole hand in my pocket with a couple of Hot Hands pads in there. Nice and toasty.

Aside from that fact, wouldn't a mitt like that prevent you from anchoring properly?