Lightly brush grill rack with vegetable oil. Then prepare the grill with medium-hot coals, or heat a gas grill. (See Note)
Slice each lemon into 6 thin rounds. Pick out seeds and discard. Arrange the slices in a single layer on a microwave-safe plate. Cover the plate and microwave the slices at full power for 1 minute. Set the slices aside to cool.
Place chicken between sheets of plastic wrap. Pound with meat pounder or bottom of heavy skillet to even thickness. Season with lemon pepper and salt.
Grill chicken breasts until firm and browned, about 4 minutes per side; an instant-read thermometer inserted in thickest part of breast should register 160 degrees F. Remove to platter and keep warm.
Sprinkle lemon slices with sugar and chili powder. Grill until browned on both sides, for about 40 seconds per side. Place 2 lemon slices on each chicken breast to garnish, and drizzle with any juice remaining in microwave dish. Makes 6 servings.