What I see as the real problem with brake wash downs is that your not washing it after you launch. You launch the boat and park the truck and trailer where the salt goes to work on it while your out for the day. Then you put it back in the salt to load the boat and the salt goes to work some more till you get home to use the washdown.
So depending on how your boat's set up I say adding a freshwater tank to your boat or truck and use the boats washdown pump to wash the brakes. All you would need is a tank, either portable to carry in the truck or one mounted in the boat, and a valve for the pump to draw raw or fresh water.