I use a similar technique. Not sure if this works on dual axle trailers. Mine is single. Lower the nose of the trailer all the way (possibly even digging a little hole to get it lower). Brace just behind the bunks. I built a frame out of 2X4s so I could do this easly every year. Crank the nose back up all the way. All the weight of the boat is now on the brace and the front roller, and the boat has lifted off the bunks about 4" This is enough that I can get bottom paint on. If I needed to sand, I have seen some that have braced the boat with long athwartships boards and pulled the trailer out from under.
Actually doing all this kind of scares the crap out of me. It all seems reasonable until you are out there balancing your boat on some scraps of wood!