Preheat oven to 350 degrees F. For crust, in a large bowl, combine flour, oats, brown sugar, and baking soda. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Remove 1 cup of the oats mixture for topping; set aside. Press the remaining oats mixture evenly into the bottom of an ungreased 13x9x2-inch baking pan. Set aside.
For filling, in a small bowl, stir together pineapple topping and rum extract. Spread pineapple mixture evenly over crust.
For topping, stir coconut into the reserved oats mixture. Sprinkle evenly over filling. Bake about 30 minutes or until top is golden. Cool in pan on a wire rack. Cut into bars. Drizzle with melted chocolate. Makes 48 bars.