Preheat oven to 400 degrees. Place bacon in a single layer in two 11-by-17-inch rimmed baking sheets. Cook in the oven until browned and crisp, turning over halfway through cooking time, 15 to 20 minutes. You may need to drain off some of the fat about halfway through cooking time. Transfer bacon to a paper-towel-lined pan to drain; discard fat.
Place avocadoes in a bowl with lime juice, and mash with a fork until chunky but spreadable.
Using a large serrated knife, split focaccia horizontally. Spread bottom with avocado mixture; layer with bacon, oven-dried tomatoes, and lettuce. Spread remaining focaccia with mayonnaise, if desired, and top sandwich. Cut into 12 squares, and serve.