Preheat oven to 450 degrees F. Halve each grapefruit; cut a very thin slice from the bottom of each half so grapefruit will sit flat. Use a grapefruit knife or other small knife to cut around the outer edge to loosen fruit from shell. Cut between each segment to loosen it from membrane.
In a bowl combine orange sections, banana slices, prunes, and if using, liqueur. Mound the orange mixture atop the grapefruit halves. Dot each half with margarine or butter. Combine brown sugar and cinnamon; sprinkle over fruit mixture. Place in a 3-quart rectangular baking dish. Bake, uncovered, for 12 minutes or until grapefruit is warm and topping is hot. Makes 6 servings.