Grate rind and squeeze juice from lime to measure 1 teaspoon and 1 tablespoon, respectively. Combine rind, juice, mint, and rum in a large zip-top plastic bag. Add steak to bag, and seal bag. Marinate in refrigerator 30 minutes, turning occasionally.
While steak marinates, prepare Pico de Gallo.
Remove steaks from marinade, discarding marinade. Sprinkle steaks with salt and pepper. Place steaks on a grill rack coated with cooking spray. Grill 2 minutes on each side or until desired degree of doneness. Let stand 3 minutes. Cut steak into slices. Serve with Pico de Gallo.