Preheat oven to broil. Place chicken breasts on a sturdy surface and cover lightly with plastic wrap. Pound chicken breasts with a meat mallet or rolling pin to flatten to 3/4-inch. Uncover, pat dry with paper towels, and generously season both sides with salt and pepper. Heat medium, oven-safe pan over medium-high heat. Add bacon and cook 3 to 4 min., or until lightly browned and crispy. Transfer bacon to paper towel-lined plate, leaving fat in pan. Cut bacon into bite-size pieces. Add chicken breasts to the same pan on medium-high heat. Cook until golden brown, 5 to 6 min. per side, or until chicken reaches 155 to 160 degrees. Spread barbecue sauce evenly over each breast and top with Monterey Jack cheese and pineapple. Place pan under broiler for 3 to 4 min., or until cheese is melted, bubbling, and beginning to brown. Top with bacon, tomato, and scallion. Serve hot.