As long as the tree was not a walnut, hickory, or pecan (a species with Juglone,) this could be a great way to use the stump space. Keep an eye on the thirst level of the Brug, especially this year. Since you'll (I hope) be making sure the Brug stays moist, If you pile mulch &/or other organic matter around the sides of the stump, it should rot-out quickly. That may or may not be a good thing, if you're enjoying the stump as a planter more than you would enjoy not having a stump at all.
