Stir together both kinds of mayonnaise, mustard, Old Bay seasoning, hot-pepper sauce, salt and lemon juice in large bowl. Add shrimp, tomato, bacon and scallion; toss to mix. Cover; refrigerate.
Cut loaf diagonally into 12 slices. Place slices on baking sheet. Coat top side of slices with cooking spray. Sprinkle slices with half the Parmesan and black pepper.
Broil 4 inches from heat for 1 to 2 minutes, watching carefully, or until cheese is melted. Flip slices over. Repeat with spray, remaining Parmesan and pepper. Broil 1 minute.
Spread 6 leaves of lettuce on platter. Tear remainder; spread on top along with shrimp. Serve with toast. Makes 6 servings.