In theory the other chloride salts would work, but magnesium forms crystals with sulfate at much higher recrystallization pressure so would be more damaging to soft stone in splash-out. Also, it isn't just the sodium that is a problem for plants but the chloride as well so you don't really solve the fundamental issue. If you are going to be backwashing into an area with more sensitive plants, just dilute by using additional water and not use only backwash for that area.