1. Place first 12 ingredients, except scallion garnish, in a food processor and puree until almost smooth. Transfer half of puree to a small bowl and keep chilled, covered, for sauce.
2. Place remaining puree in a large heavy-duty resealable plastic bag with chicken and seal, turning to coat well. Marinate, chilled, turning bag occasionally, at least 8 hours or overnight, placing bag in a large bowl to catch any leaks.
3. Preheat grill and lightly oil rack. Let chicken stand in marinade at room temperature 30 minutes before grilling. Meanwhile, cut sides and ends off pineapple. Cut crosswise into 15 slices about 1/4 inch thick. Sprinkle one side of each slice with 1/2 teaspoon packed light brown sugar. Place sugared side down on rack and grill, turning once, until golden brown, about 3 minutes. Transfer to a platter, alternating slices with scallion garnish.
4. Remove chicken from marinade (discard marinade) and grill over moderate heat, turning occasionally, until cooked through, 35 to 45 minutes, depending on size. Transfer to platter with pineapple and serve with remaining sauce on side (exercise caution; sauce is spicy). Makes 6 servings.