Naomi

The anus is notoriously difficult to repair and once ruptured it's a disaster. So much so that it is the reason that in the UK episiotomies ( the labial cuts performed during birth so the labia don't tear ) are done diagonally on one side so that if they extend they don't intrude on the anus. In the US the cut is done in the midline at the back of the vulva compromising risk of anal tearing for a better cosmetic result afterwards.

A temporary bypass of anal continence seems a little harder. Theoretically you could do one sphincter with bo-tox but that's ludicrously extreme. A hollow butt-plug and laxatives perhaps?

A lot easier and almost as humiliating/disabling for the slave would be to remove urinary continence. You can bypass your bladder control simply by inserting a catheter ( sterile technique ), you could cut it short ( at increased risk of ascending infection ) and you'll happily leak any mess your body produces all over the floor/your pants...etc..

Try that one at the roadside