Generously season the ribs with salt and pepper, then rub with the vegetable oil and sprinkle with the chile powder, pressing to adhere. Whisk 1/3 cup barbecue sauce, the vinegar, ginger and garlic in a 6- to 8-quart slow cooker. Add the ribs and turn to coat in the sauce. Cover and cook on low until the ribs are tender, 7 hours.
Preheat the broiler. Transfer the ribs to a rimmed baking sheet. Remove 1/4 cup of the cooking liquid to a small bowl and whisk in the remaining 1/3 cup barbecue sauce, the pineapple preserves and ketchup.
Brush the pineapple sauce all over the ribs to coat completely; arrange bone-side down. Broil until the sauce is sticky and the ribs are lightly charred in spots, 3 to 5 minutes. Top with the scallions.