Preheat smoker to 250°F with fruitwood. Set a wire rack over a sheet pan.
Trim pork belly if needed, then cut into even 1.5-inch cubes.
Toss cubes with yellow mustard until lightly coated.
Stir brown sugar, salt, pepper, paprika, chili powder, garlic powder, onion powder, and cayenne to make the rub.
Season pork belly generously with the rub on all sides. Arrange on the wire rack with space between pieces.
Smoke for about 2.5 hours, spritzing with apple juice every 45 minutes, until cubes have a set bark and reach around 165°F.
Combine barbecue sauce, maple syrup, bourbon, butter, vinegar, Worcestershire, honey, and a splash of apple juice in a foil pan. Add pork and toss to coat.
Cover the pan tightly with foil and return to the smoker. Cook 1.5 hours until very tender, 200–205°F.
Uncover and cook 20–30 minutes, stirring once, until the sauce reduces and turns glossy.
Rest 10 minutes before serving to allow the glaze to set.