Flip ribs bone-side up, remove silverskin membrane with a knife and paper towel. Pat dry. Mix mustard, 1/4 cup apple juice, and Worcestershire; slather on both sides. Coat with dry rub. Refrigerate 1 hour (or overnight).
Preheat pellet grill to 180-225°F with apple or cherry pellets. Place ribs meat-side up on grates, smoke for 3 hours, spritzing with apple juice hourly if desired.
Increase grill to 225°F. Lay out heavy foil, place ribs meat-side down, add half the brown sugar, honey, butter, and remaining apple juice. Seal tightly, grill 2 hours.
Unwrap ribs, discard foil and juices. Brush BBQ sauce on both sides. Grill meat-side up for 1 hour until sticky and internal temp reaches 195-205°F (bend test: ribs crack slightly when lifted).
Rest ribs under foil for 10-15 minutes. Slice between bones, serve with extra sauce.