You can stand over the bowl in a door-less stall and pee, and all they'll see is your back, maybe your ass cheeks so what. But, with your back facing 'em, you could use the most basic, bright--orange, workable STP and they'd not see that.
Heck come to think of it, you could have an STP that you just have a sort of fake 'head' on, since your hand would cover most of it, maybe make a head out of FIMO or something.