Like many of you I also used to penetrate the anus.
Most times, it was fine, but a few times, it released the mess from hell. I just don't need that trouble in my day job, and I don't charge enough.
So not I say no to all requests from "prostate massage" or even a request to touch anus.
I am all for keeping my work easy and simple.
But I would say in the past when I did that, it would be over a spare towel with lots of good quality PLENTY kitchen roll very handy, so I was able to control and capture the shit eruption before it went out of control.
If you do it, I recommend being prepared for the worst.