1. Preheat grill to medium-high heat.
2. Thread 4 chicken cubes alternately with 3 tomatoes, if desired, onto each of 8 (12-inch) skewers. Sprinkle skewers with 1/2 teaspoon salt and pepper; coat with cooking spray. Place skewers on a grill rack coated with cooking spray; grill 3 minutes. Turn skewers over; grill 2 minutes or until chicken is done.
3. Grate 1 1/2 teaspoons rind from 1 end of lemon; cut lemon in half, crosswise. Cut ungrated lemon half into quarters; reserve remaining lemon half for another use. Combine the remaining 1/4 teaspoon salt, lemon rind, garlic, and oregano in a food processor; process until finely ground, scraping sides. With motor running, slowly drizzle oil through the food chute; process until well blended. Brush oregano mixture over skewers; serve with lemon wedges.