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.
Good luck,
Pat