First thing I would do is open the door to your house and turn up the furnace...unless you have electric heat...
Freezing rain coating your GH will, in effect, form an insulating blanket, unless it melts, then all bets are off.
Put some water
in the greenhouse.
Lots of it!! Fill up every cooler, bucket, storage container, and old milk jug you can find and spread them around inside the GH. It will keep the temps up as it will take longer for the GH (and water) to get to the freezing point. It will also protect the corms in containers if put around the containers. I have been collecting old milk jugs to line my GH back wall just for this reason. I'll put shelves for them to sit on and I will not have to go the heater route. Of course I don't have to deal with extended freezes out here...
You might also want to try one of the catylytic propane heaters you can get from you local Lowe's or Home Despot. It clamps to the top of the propane tank and will keep a 10x12' greenhouse above 50 degrees with no problems and might push the temps higher. My local HD is selling propane refills for under 17 bucks. Add a
small battery powered fan like they sell for RV's that are powered by a 12 volt battery, running on
low for circulation to eliminate the coldest spots.
If you don't have a lot of extra GH space, use a propane powered turkey fryer full of water. I brew beer on a propane fired turkey fryer base that holds a 15.5 gallon beer keg with the top cut out as my brew kettle. Bring the water to a boil
in the greenhouse every evening and then just shut off the propane burner and walk away. Heck, just the heating rpocess will warm up the GH, but the hot water will keep a greenhouse warm all night. Bring it to a boil every night til your power comes back up. Heck, It might just get you into homebrewing...
The sun will help you alot thru the day. Leave the door open to let the GH air out for about 5-15 minutes during the day to drop the CO2 concentration if it worries you or you like to tend the bananas and will spend some time out there, but the plants will do fine with elevated CO2 (3-5X as much as in the atmoshere) and actually grow faster... Just remember that CO2 sinks and will be in higher concentration at the floor. The circulation fan will help keep it more even up by the leaves.
You don't need to try to keep the temp up to 70, just above freezing and preferably above 50 degrees will do it til the emergency is over and you can resume your electric heater.
Hope some of this helps someone...good luck....
Jeff
P.s. If your power is already out, then I don't think you'll get this message...