So alot of people when they get tongue piercings will play with it with their teeth. So i got this idea that what if Deku got a tongue piercing so that he noticed when he was muttering. He could then literally hold his tongue via the piercing with his teeth.
I also think the idea of piercinfs or tatoos go along with Dekus need for pain or to feel punished for being worthless or burdening people as its something mildly acceptable that he could do for widespread pain (tatoo) or small concentrated pain (piercing) that he can tug on when he needed to.
I still imagine alot of the piercings in unobtrusive places so they dont get in the way during a fight or are easily noticed.
So like everyones shocked when deku has a full thigh tatoo or a tongue piercing.