My podiatrist insists on twice-daily Epsom salt soaks after toenail surgery. It seems to improve healing; it definitely has some noticeable effects (namely, improved “regularity” and improved sleep). The medical literature indicates that there is little or no evidence from studies to support the idea that you can absorb enough Magnesium, transdermally, from Epsom salt soaks to make any difference at all, but it certainly has plenty of anecdotal support.

To my way of thinking, if it works, it works — as you point out, the risks of absorbing too much Magnesium this way are incredibly low, and I think you’d notice gastrointestinal distress and back off before it became a problem.