Heat outdoor grill or a grill pan over medium-high. Coat corn with cooking spray, and place on grill grate (or pan). Grill, turning until charred on all sides, 12 to 14 minutes. Place on a cutting board; let stand 5 minutes or until slightly cooled. Cut kernels from corn into a bowl.
Place crabmeat, pepper, 2 tablespoons lemon juice, and 1/4 teaspoon salt in a large bowl. Toss to combine.
Brush oil evenly over cut sides of lettuce hearts; place 3 lettuce halves, cut side down, on hot grill or grill pan. Grill until charred, about 1 minute, watching closely to prevent burning. Repeat procedure with remaining romaine hearts.
Stir together guacamole, remaining 1/4 cup lemon juice, and remaining 1/4 teaspoon salt.
Place 2 romaine halves, charred side up, on each of 6 plates. Spoon 2 ounces crabmeat mixture evenly over top. Top evenly with corn and tomatoes. Spoon guacamole dressing evenly over top of salads.