There was a thread a while back about using skillfully placed tongue rings connected to various creative other body parts for keeping the tongue forward and out of the way. (the tongue-in-cheek thread???)
I don't recall any serious posts that suggest using the body-piercing technique for keeping the MOUTH shut. The engineering side of my brain asks, WHY NOT?
I can see maybe a couple of rings on the lower lip, and one through the nose in the regular place. At night place a teeny-tiny bungee cord (or something) to connect the three which would apply a force field in just the right spots to encourage keeping the mouth in the "closed" configuration. Any attempt to compromise the position would send negative feedback signals to the pain center of the brain, which would result in a "rapid learning" response, and the desired result. OPERANT CONDITIONING
might kick in and cause great resistance to any impulse to open the mouth, even without said appliance, including actual perceived pain, yelling and screaming, etc.
No matter how stupid you think it may look, I bet nowadays nobody would even notice a modest few rings here and there on your face, tastefully placed. Or for the overly self-conscious, get the removable kind, just wear them at night.
Another serious benefit, when kids see OLD PEOPLE with body piercings and rings, they will abandon the practice outright. That has got to be good.
He who dies with the most masks wins.