Prepare the Sauce: In a large mixing bowl, whisk together the pineapple juice, ketchup, brown sugar, Worcestershire sauce, garlic powder, ground mustard, and cayenne pepper to taste until smooth. Reserve 3/4 cup of the sauce in a small bowl for later glazing.
Marinate the Chicken: Season the chicken generously with salt and black pepper. Place the chicken in the remaining sauce and toss to coat evenly. Cover the bowl with plastic wrap and refrigerate for at least 30 minutes, up to 2 hours.
Arrange for Roasting: Position a rack in the center of the oven and preheat to 425°F (220°C). Line a baking sheet with foil and arrange the chicken skin side up. Scatter the pineapple rings around the chicken pieces.
Roast and Glaze: Roast the chicken for 30 minutes. Brush the chicken with half of the reserved sauce, then continue roasting until the skin is crispy and caramelized and an instant-read thermometer inserted into the thickest part of the chicken registers 165°F (74°C), approximately 10 more minutes. For a finishing touch, turn on the broiler and broil briefly, watching closely, until the skin begins to darken, about 3–4 minutes.
Finish and Serve: Transfer the chicken to a serving platter. Brush with the remaining sauce and garnish with sliced scallions and jalapeños.
Make-Ahead Option: The BBQ sauce can be prepared up to two weeks in advance. Store in an airtight container in the refrigerator until ready to use.