1. Preheat the grill or broiler.
2. In a large bowl, combine the beef, barbecue sauce, chili powder, cumin, garlic powder and onion powder, and mix with clean hands until well blended.
3. In a small bowl, mix the dressing and mustard together until smooth.
4. Divide the beef mixture evenly into four ¼-pound burgers. Grill the burgers for 5 minutes on each side over medium coals for medium doneness.
5. Place the pitas on the grill for 2 minutes or just until warmed through.
6. Serve the burgers tucked inside each pita and topped with the tomato, lettuce and a generous serving of the dressing.