Frostbite is caused by bellow freezing temps, not moisture or humidity... Moisture and humidity do drastically increase the risk of frostbite at moderate freezing temps (as does wind or drafts on exposed skin) and thus it's important to lower the humidity if possible to help alleviate that risk...
I don't know how much you want to invest, but a portable car port placed over the coop and wrapped on the prevailing wind side with a tarp to create a wind block should eliminate most of your issues...
http://www.harborfreight.com/10-ft-x-20-ft-portable-car-canopy-60728.html
Just make to sure...