Thanks for raising this. It had never crossed my mind before. I will have to do some research and find out if there is any data on the effect of bath salts on the skin. It is amazing how things right under your nose often fail to get your attention.
My first thought is that the ideal bath salt should make the bath isotonic. By that I mean have the same osmotic pressure as your skin so that it neither draws water out nor pushes water into the upper layers of the skin.
I will report back if I can find anything interesting.