Preheat oven to 250°F. Season pork belly all over with 4 teaspoons salt and 2 teaspoons pepper. Place chiles in a large roasting pan. Place pork belly, skin side down, on top of chiles; drizzle with 1/2 cup agave. Pour remaining 1/2 cup agave and 2 cups water into pan. Cover pan tightly with lid or two layers of foil. Transfer to oven and cook pork for 3 hours, basting with pan juices every hour. Turn pork over. Cook until very tender, about 4 more hours. Transfer pork to another roasting pan (if bone-in, remove and discard bones). Strain liquid from first roasting pan into a saucepan (there should be about 5 1/2 cups); reserve chiles. Gently boil juices until reduced to about 1 1/4 cups, 45-50 minutes; add reserved chiles. Remove pan from heat. Let glaze cool; chill. Meanwhile, cover pork with parchment or waxed paper. Top with a large plate. Place weights, such as large canned goods, on top. Chill overnight. Reheat glaze. Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Brush grill rack with oil. Grill pineapple until nicely charred, 2-3 minutes per side; place on plates. Let grill cool to medium heat. Slice pork belly into 1"-thick pieces. Grill until warmed and crisp, about 3 minutes per side. Brush pork with some of the glaze from chile sauce. To serve, place 2 or 3 pineapple slices on each plate; spoon glaze over, then top with pork belly. Drizzle with vinegar.