Just a thought...what about putting a heated rock or brick in the waterer? I don't know how well it would work, and I know rocks CAN explode, so you would have to use caution...but it may work. I am envisioning putting it inside a five gallon galvanized waterer perhaps?
Not sure how long it would stay warm being submersed though-I know we used to heat them in the campfire when we went to the ocean, wrap them in a blanket and put them in our beds...and they were still really hot int eh morning- did a great job for warming the tootsies!
I am curious now...as I dont have a heated base. Was plannign on getting one, but maybe I will give this a try- perhaps putting a couple of hot bricks UNDER the waterers base....would need some sort of protective barrier around it to protect chickens from any exploding rock fragments of course....
I very sincerely do not think there is much worth doing except just bringing water out repeatedly during the day. For other animals, you can float a partial cover on the water surface to reduce heat loss, but I betcha chickens would just try to jump on and would get all wet and chilled plus messing up the water; plus which, chicken waterers are so SMALL to begin with that you would not be gaining much anyhow.
Some like to use fortex (rubber) pans for water so that ice knocks out easily.
I have tried putting hot bricks in horse buckets to delay freezing. It did next to no good. I doubt it'd work much better for chickens. The danger of exploding is when a damp brick is being heated up, not afterwards, btw.
You'd need a fairly huge solar system to run an electric waterer heater (heating things up draws a lot more watts than just, say, a lightbulb); the only passive solar that's going to work well is simply putting your waterer in a spot in the coop that gets sun most of the day.