Prepare grill for direct-heat cooking or position broiler so that top of steak on rack in pan will be 4 inches from heat source; heat broiler. Combine barbecue sauce, lime zest and 1 t. lime juice; reserve. Rub both sides of steak with seasoning and salt.
Grill or broil steak, turning once, 3-5 minutes per side for medium-rare; sprinkle with remaining 1 t. lime juice and brush with reserved sauce mixture during last minute of cooking. Let stand 5 minutes before slicing. If desired, garnish with lime and parsley and serve with additional barbecue sauce.