In a medium bowl, combine chicken, bacon, mayonnaise, celery, green onion, and 1/4 teaspoon pepper; cover and chill.
Using a serrated knife, cut a thin slice off bottom of tomatoes to create a level base. Cut tops from tomatoes. Using a melon baller, remove pulp, leaving shells intact. Turn tomatoes upside down on paper towels to drain for 10 minutes. Season inside of tomato shells with salt and remaining 1/4 teaspoon pepper.
Spoon chicken salad into each tomato shell. Garnish with lettuce and bacon, if desired.