Prepare muffin mix according to package directions for corn bread, using fat-free milk. Let cool; crumble and set aside.
Coat a large nonstick skillet with cooking spray. Place over medium heat until hot. Add sausage, onion, and celery; cook until sausage is browned and vegetables are tender, stirring until sausage crumbles. Drain and pat dry with paper towels.
Combine corn bread, bread cubes, sage, and pepper in a large bowl; stir in sausage mixture. Add chicken broth and egg substitute, stirring well. Spoon mixture into a 13- x 9-inch baking dish coated with cooking spray. Reduce oven temperature to 350°, and bake, uncovered, for 1 hour or until browned. Serve warm.
Tip: Look for packages of corn muffin mix (such as Martha White) that are prepared with only milk or water.