Magnesium (Mg) will combine with fatty acids to form a magnesium soap which is insoluble in water. Magnesium soap is one component of "soap scum" -- the sticky, greasy, icky coating on your skin if you wash with soap in hard water. I imagine that's why epsom salts added to CP soap doesn't work well.
We've been having a related discussion in another thread -- the OP asked about putting magnesium chloride (MgCl) "oil" in soap. The question in that thread was this -- can MgCl "oil" be added to CP soap batter? Or does the solution of MgCl and water (aka magnesium "oil") have to be added after the cook in HP soap?
The consensus in that thread was adding MgCl to CP soap is a great way to make an icky magnesium-based soap. I'd say adding epsom salts (MgSO4) to CP soap will do likewise. The OP in the magnesium "oil" thread has been adding the "oil" to HP soap with some success, however. You could try a test batch and add epsom salts to an HP soap after the cook. That might give you an acceptable "epsom salt" salt bar, but that's just a guess -- I personally have not tried it.
