I know from first hand experience you can't do it by accident as one time, hung over and showering in the dark in the early morning i got a little dizzy and sat down.
Perfectly meeting my starfish to the shampoo bottle on the floor. It could not have been more on target had i attempted this.
I shot up, seeing stars like for real for only the second time in my life. I wasnt dizzy, I was up and at em' baby. Wooo what a rush! Hurt a lot for a minute. Hard as i sat down I don't think that bottle tip made it a millimeter into my pooper.
So yeah, impossible
Cool, what others are there? Also, goofy question: what's the relative strength of this 'outer anus'? It wasn't listed in my search for strongest muscles and most results are tips on gaining anus strength