Step 1In a large bowl, whisk pineapple juice, ketchup, brown sugar, Worcestershire, garlic powder, ground mustard, and cayenne to taste until combined. Transfer 3/4 cup sauce to a small bowl; set aside for glazing.

Step 2Season chicken all over with salt and black pepper. Add to bowl with remaining sauce and toss to coat. Cover bowl with plastic wrap and refrigerate at least 30 minutes and up to 2 hours.

Step 3Arrange a rack in center of oven; preheat to 425°. Arrange chicken skin side up on a foil-lined baking sheet. Scatter pineapple rings around chicken.

Step 4Roast chicken for 30 minutes. Brush with half of reserved sauce. Continue to roast until skin is crispy and caramelized, and an instant-read thermometer inserted into thickest part of chicken registers 165°, about 10 minutes more. Turn on broiler and broil, watching closely, until skin is starting to blacken, 3 to 4 minutes.

Step 5Transfer chicken to a platter. Brush with remaining sauce. Sprinkle with scallions and jalapeños.

Step 6Make Ahead: BBQ sauce can be made 2 weeks ahead. Store in an airtight container and refrigerate.