Very little. Magnesium is far down the list of body anions (after Sodium, Potassium and Calcium). If you want an isotonic solutions, you want it to be isotonic for the most common ions, such as Sodium,potassium, Chloride and Bicarbonate. The point of using an isotonic solution is so there is no signficant loss of electrolytes. As a result a solution that is isotonic for magnesium only would be pretty useless
Something on the order of 5 grams of Epsom salts per liter would make the solution isotonic for Magnesium, but hypertonic for Sulfate, and hypotonic for sodium, potasium, bicarbonate and chloride.
The normal laxative dose for epsom is hypertonic for magnesium, and as a result, draws water into the bowel lumen by osmotic pressue.
If you want a truly isotonic solutions for most of the common ions, forget about Magnesium. In various books are the formulates for products like Golytely (NOT Nulytely). Take out the PEG 3350 from the formula, and those solutions are isotonic for sodium, potassium, Chloride, Bicarbonate and Sulfate.