Combine first 4 ingredients in a small bowl; make a well in center of mixture. Combine buttermilk, 1 tablespoon egg substitute, and oil; add to cornmeal mixture, stirring just until dry ingredients are moistened.
Pour batter into a 6-inch cast-iron skillet coated with cooking spray. Bake at 450° for 20 minutes or until golden. Remove from skillet, and cool completely.
Coat a small nonstick skillet with cooking spray; add margarine. Place over medium-high heat until margarine melts. Add celery and onion; saute 4 to 5 minutes or until tender.
Crumble cornbread into a bowl. Add vegetable mixture, bread, and remaining ingredients; stir well. Spoon mixture into a 7- x 5- x 1 1/2-inch baking dish coated with cooking spray. Bake at 325° for 40 to 45 minutes or until golden.