Okay, Texas, I understand better that some of you don't have coops that are enclosed. Here is a trick to help, I hope...
If you have hay or straw bales, build a little house for them to get you through the cold spell. You shouldn't need a heat lamp anyway if it is still above zero. Make their new house two bales high if you can and put something in it they can roost up off the ground one, even if it is only a 2X4. Put that ginormous plywood on top for a roof. If your birds are DRY and it is not too drafty, they can get through with that as far as shelter goes. Like someone else suggested, make sure they have water. Up her on the lower tundra we use water heaters that you set your waterer on and it keeps it thawed, but since you probably wouldn't get much use out of it, you may be able to come up with something else that will work. I have heard of people putting a light (single incandescent type) inside of a terra cotta pot turned upside down and then setting water pan on that. Sounds kind of tricky and I wouldn't do it in a strawbale house. You will see your birds probably look fatter, they will fluff themselves up to better insulate their heat (something I wish I could do). I would think that unless you have those breeds with huge combs and wattles, you shouldn't need to worry over frostbite or putting on vaseline. You won't have that long of a spell, will you?
I don't mean to sound like a smarty pants, but sitting in an old 3-story farmhouse that isn't exactly buttoned up real tight for the -15 wind chill that is screaming outside, it is making me chuckle a little. Some days in July it does get up to 98˚ here and you would laugh at me if I were complaining about the heat, right? I say this, without A/C in this place at those times, you can only take off so many clothes before you start scaring people, so I might complain a little.
I hope your temps get back to normal soon. good luck y'all.
