I don't think you would get anywhere near enough pressure with just body weight on a large bladder. The weight would be distributed over too large an area. There must be some reason why no one has designed a simple bladder type reverse osmosis system yet. Perhaps the pressure required is so great that it would burst the bladder(imagine standing on or jumping on a hydration pack-I imagine something like that would be required to generate enough pressure-which of course you couldn't do on a life raft anyway).