For the five-spice powder: Toast the fennel, Sichuan peppercorns, white peppercorns, cloves, star anise and cinnamon in a dry, hot wok until fragrant. Remove and let cool slightly. Grind the spices to a powder in a spice grinder. (The spice mix can be kept for up to 1 month in an airtight container at room temperature.)
For the roast pork: Hang the pork belly on a meat hook set over a sink. Pour boiling water over the skin of the pork and let it drain. (Avoid pouring hot water on the meat and fat.) Pat the pork dry.
Brush the pork skin with the vinegar-sugar mixture. Refrigerate on a wire rack, skin-side up, uncovered, for least 10 hours.
Preheat the oven to 450 degrees F.
Score the meat-side of the pork belly 1/2-inch deep. Mix together the five-spice powder, salt and sugar and rub the meat (but not the skin) with the mixture, making sure to get it into all the crevices.
Pierce the pork skin all over using a pork skin tenderizer or a sharp knife; do not pierce the meat. Set the wire rack into a roasting pan and fill the pan with 1 inch of cold water. Place the pork skin-side up on the rack and roast for 45 minutes. Reduce the heat to 350 and roast 2 hours more. Raise the heat to 400 and roast until the skin is nice and crispy, 15 minutes. (Alternatively, finish by broiling until the skin gets bubbly and crisp.) Reserve the remaining liquid in the pan.
Finely slice the pork belly for service, leaving the skin on. (Or separate the crispy skin and chop it into fine pieces; use as garnish.)
For the tropical salsa: In a bowl, combine the pineapple, cilantro, mango, red chile, jalapeno, red onion, scallion, lime juice, orange juice, salt, sugar and white pepper to taste. Mix until well blended. (Use within 2 hours; makes about 2 cups.)
For service: Heat the hoisin sauce in a small saucepan or a wok until bubbling; whisk in 1/4 cup of the reserved pork cooking liquid and cook until the sauce thickens and turns glossy. Taste and add salt or more cooking liquid as necessary.
The sliced pork can be served family-style over a bed of the tropical salsa, with the hoisin sauce on the side and steamed Peking duck pancakes for wrapping. (Or the pork can be served on flash-fried wonton wrappers brushed with the hoisin sauce, and garnished with the tropical salsa.)