Preheat oven to 400°.
To prepare fruit mixture, combine the first 5 ingredients in a saucepan. Bring to a boil; stir occasionally. Reduce heat; simmer 10 minutes or until cranberries pop and mixture is slightly thick. Cool slightly; stir in apple and rum. Spoon into a 3-quart casserole coated with cooking spray.
To prepare topping, lightly spoon flour into a dry measuring cup, and level with a knife. Combine flour, 1/4 cup sugar, baking powder, baking soda, and salt. Cut in margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Combine buttermilk and rind, and add to flour mixture. Stir just until moist.
Spoon topping into 8 equal portions over fruit mixture; sprinkle with 2 teaspoons sugar. Bake at 400° for 35 minutes or until filling is bubbly and topping is golden brown.