Preheat the oven to 375 degrees F. Spray a 9- by-13-inch baking dish with nonstick cooking spray.
Add the cornbread, parsley and rosemary to a large mixing bowl. Heat 4 tablespoons of the butter in a large skillet over medium-high heat. Add the celery, green onions and bell pepper to the skillet and saute until softened, about 10 minutes. Add the remaining 4 tablespoons butter and melt. Add the cooked vegetables and melted butter into the bowl with the cornbread and stir to combine. Pour the milk, oysters and reserved oyster juice over the cornbread, then season with the garlic powder, 2 teaspoons salt and some pepper. Pour in the chicken stock and eggs and stir until well combined and cohesive. Spread into the prepared baking dish.
Cover the dish with aluminum foil and bake, about 30 minutes. Uncover and bake until the top begins to brown, about 30 minutes more.