We have two sets of muscles to control the exit of waste from the anus. One is smooth muscle tissue, and not under conscious control. It opens as a reflex when it feels pressure. Just as all sphincters do in the digestive track.
The second is skeletal muscle. It is under conscious control and is what we learn to control when we get potty trained as kids.
With that said, I have several concerns regarding a procedure such as this.
One, you probably won't be able to find a reputable and safe doctor willing to do it. That type of "elective" surgery is not only dangerous but doesn't even serve any cosmetic value. Doctors aren't too quick to completely mess up the function of a vital body part without a damn good reason.
Two, while muscle tissue does have the ability to heal, how well it does it depends on how the muscle is cut. With or across the grain of the muscle tissue. If you go with the grain, then the muscle has a much greater chance of healing without scar tissue and without deteriorated function. If you cut across the grain, you are severing the muscle cells and it is much harder to heal.
Third, you are dealing with an area ripe with bacteria. Any bacteria near a wound is not a good idea. Let alone having a wound in an area that manufactures bacteria as part of its natural function.
Fourth, while this is not a biological concern per se, it is a valid thought. Fantasies are great. And many fantasies should remain just that ~ fantasies. This sounds like one that should remain in that realm.