Saltwater freezes at a colder temperature than normal water. It does not stay warmer, just does not freeze at the same temperature. (Of course you shouldn't give saltwater to the chickens to drink.)You can try the saltwater trick and floating a water bottle of saltwater in the dish to help it move and break up the ice. Something else you could try would be to insulate the dish from the ground itself, such as on a wood or stone block.
If the saltwater is going to be in a water bottle to break up ice, not mixing with the drinking water, then it doesn't matter whether there is saltwater or normal water or any other kind of weight in the bottle. Anything that moves around would have the same effect.
Moving water does not freeze as fast as still water, but even big bodies of water (like oceans) and fast moving water (like Niagara Falls) can freeze when the temperature gets cold enough. So moving the water around, and breaking the ice on the surface to expose the liquid underneath, will only help in certain temperatures. In very cold weather the dish will just be full of ice by morning, solid all the way down, unless there is something providing actual heat.