When I went dogsledding in Ely, MN, in Jan the outfitter set me up with some warm gloves and some super mittens. The mittens have a strap just like little kids' have--run it up your sleeve, over your back, and down the other sleeve of your coat. They go most of the way up my arm. The strap is what makes them killer: when I need dexterity, I just pop them off and they dangle out of the way while I work with my gloves on. Then, I can slip my hands back in them for driving the tractor. They worked dogsledding and they worked plowing snow in our Post-Zombie-Apocalyptic Winter of 2008-2009.
The thing I found with my gloves is when I took them off the change shear bolts or something, they were tough to get back on with wet hands.
Rob