I did not use epsom salt on P2, but I have seen it used orally for constipation and in detox baths.
Here is a good link about detox baths (some ladies on here do an apple cider vinegar bath).
http://www.wikihow.com/Take-a-Detox-Bath
Just remember no oils in P2
And as far as the epsom salt for constipation, 1-2 tsp in warm water. Doesn't sound very fun at all. I liked to use lemon juice (about 1/2 fresh lemon) in water with a little stevia. It will keep you regular and actually tastes good!