There already has been pluggers made for md'rs over the years. The "hole hog", etc... I would assume there still is, although I'm not near a kellyco or any other detecting catelogs to see. They took flack, for the reason Dave alludes to: They tend to leave a dead brown ring, because you're not leaving a "flap" in place. Technically a 3-sided plug is better, because that leaves root systems intact, making it less likely for the plug to die when you repack & stomp it. However, trying to retreive a coin with that plug in your way is sometimes a pain, especially if you have to re-scan the hole with your detector, the lump there will keep getting in the way.
Where I'm at, the greener parks (assuming a person isn't going in the height of the summer dry season), are lush and moist enough that a 4-sided plug doesn't die. So around here, I've seen several people with pluggers. I once even made one myself out of a calking gun. Now though, I just use a gator tool/knife, or a bayonette you can get from military surplus stores, to dig/cut.