In a large saucepan, combine the ketchup, brown sugar, vinegar, soy sauce, onion, celery seed, 1/4 teaspoon salt, pepper flakes and liquid smoke if desired. Bring to a boil. Reduce heat; cook and stir over low heat for 10 minutes or until thickened.
Meanwhile, rub chicken with oil and sprinkle with remaining salt. Grill, covered, over medium heat for 3 minutes on each side.
Set aside 1/2 cup barbecue sauce for serving. Brush half of the remaining sauce over chicken. Grill, covered, 7-12 minutes longer or until a meat thermometer reads 160°, turning and basting occasionally with sauce. Serve with reserved sauce. Yield: 4 servings.