Position a rack in the upper third of the oven and preheat to 425 degrees F. Coat a rimmed baking sheet with cooking spray. Combine 1 tablespoon hoisin sauce and 1 teaspoon rice vinegar in a small bowl; set aside.
Combine the pork, panko, the remaining 1 tablespoon hoisin sauce, the scallion, ginger, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl. With dampened hands, roll the mixture into twelve 1 1/2-inch meatballs; transfer to the prepared baking sheet. Bake, turning once, until browned and cooked through, 5 to 6 minutes per side. Remove from the oven and brush with the reserved hoisin sauce mixture.
Meanwhile, combine the cabbage, carrots and pineapple in a food processor. Add the remaining 2 tablespoons rice vinegar, the mayonnaise and 1/2 teaspoon salt and pulse until roughly chopped.
Spread mayonnaise on the bottom half of each bun, then fill with some shredded lettuce and a meatball. Serve with the pineapple salad.