In a bowl, whisk together the flour, baking powder, cinnamon, salt and nutmeg until combined. In a large bowl, whisk together the brown sugar, coconut oil, eggs and vanilla extract. Fold in the dry ingredients. Fold in the grated zucchini until combined and dispersed throughout. Spread the batter in the greased pan. Bake for 30 to 35 minutes, or until the bars are set and not jiggly in the center. Let bars cool completely, then frost.