Broilers vary in heat intensity; move the rack to a lower position if the pork is browning too quickly, or to a higher rack if too slowly.
Preheat broiler with rack 4 inches from heating element. Place pork and scallions on a rimmed baking sheet and toss with 1 tablespoon oil; season with salt and pepper. Add pineapple to sheet, spreading all ingredients in a single layer. Combine cayenne and 1/2 teaspoon salt in a bowl; sprinkle evenly over pineapple.
Broil until pork is just cooked through, 3 to 4 minutes. Transfer pork to a cutting board. Reserve accumulated juices in a bowl. Continue broiling scallions and pineapple until blistered in spots, 6 to 8 minutes (remove scallions earlier if blackening too much). Whisk ginger and lime juice into reserved juices. Slice pork; chop scallions and pineapple. Serve over rice with dressing and chiles.
