OP, get some chemi-warmers. They need oxygen, so don't seal them up too tightly.

If you want to experiment with stupidly warm self-heated hand covers, try Outdoor Research Alti Mitts. At REI, you can get a pair (really a quad, since there's an outer and inner mitt) and return them at any time (e.g, after a full winter's test) if they don't work for you. I rode at 14 today (-2 WC) with the outer mitts and thin North Face glove liners. These will accommodate chemi-warmers, but unless you're in Antarctica or on top of Denali or Everest, you won't need them.

http://www.rei.com/product/787066


You lose some dexterity, which you may find warrants a refund.

Epic Designs pogies of Fairbanks AK are another option, made for bare hands or thin glove controls operation inside a well insulated cavern, albeit not usable on drop bars. Moose Mitts offers a drop-bar pogie, not as warm as Epics but you aren't in central AK.