I sit on the ball and massage my piriformis. Goes nowhere near the butt crack, so you don't have to worry about explaining things to the ER doc. Oh, and do it clothed, duh!
I'm on my foam roller and my Lax ball on the butt nearly every day, but certainly after squatting and doing deadlifts. I don't have feet problems, so I don't bother with it for my feet.