Combine flour, 1/4 cup powdered sugar, and lemon rind. Cut in margarine with a pastry blender until mixture resembles coarse crumbs. Press mixture into a 9-inch square pan coated with cooking spray. Bake at 350° for 18 minutes.
Combine 1 cup sugar, egg substitute, and lemon juice, stirring well with a wire whisk. Pour over baked crust. Bake 25 additional minutes. Let cool completely on a wire rack.
Sprinkle with remaining 1 tablespoon powdered sugar. Cut into 16 bars. Store in an airtight container.