Rule books that address jewelry refer to exposed jewelry. Something worn under the uniform is the players business. As an umpire, I'm not going to know it's there- and I'm certainly not going to be checking for it!
Agree, if the dermal thing was visible (I know a woman who has one in the center of her chest) due to a v-neck or unbuttoned jersey, that would be easy to throw a band-aid over. Who would question that? I know several girls who play with clear plastic piercing keepers in their noses during games. If you put foundation on it, you can't even tell it is there. They just have them in so the hole won't close. We've got two girls on my Varsity who have tongue rings, they never come out.