Preheat a pellet grill to 250°F. Use a mild wood pellet, such as applewood, for a gentle smoky flavor. Line a rimmed baking sheet with foil.
In a large mixing bowl, combine the ground beef, egg, panko, chopped pineapple, onion, 2 tablespoons teriyaki sauce, ginger, garlic powder, and black pepper. Mix gently just until combined so the meatloaf stays tender.
Shape the mixture into a compact loaf on the lined baking sheet. Leave a little space around it so smoke can circulate.
Place the baking sheet on the grill and smoke the meatloaf until its center reaches 150°F. Check the temperature with an instant-read thermometer inserted into the thickest part.
While the meatloaf smokes, stir together the remaining 1/4 cup teriyaki sauce, pineapple juice, and rice vinegar in a small bowl.
Brush the glaze over the meatloaf. Continue cooking until the center reaches 160°F and the glaze looks glossy and lightly set.
Remove the meatloaf from the grill and let it rest for 10 minutes before slicing. Sprinkle with sliced green onion just before serving.