Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground beef, cooked rice, egg, panko, pineapple, half of the green onion, ginger, garlic, 2 tablespoons of the teriyaki sauce, and the rice vinegar. Mix gently until evenly combined. Avoid packing the mixture tightly, which can make the muffins dense.
Divide the mixture among the 8 prepared muffin cups. Press it in lightly, leaving the tops slightly rounded.
Bake until the meatloaf muffins are cooked through and reach 160°F in the center, about 20 to 25 minutes. Let them rest in the pan for 5 minutes, then lift them out carefully.
While the muffins rest, stir the remaining teriyaki sauce with any juices collected in the muffin cups. Brush the sauce over the tops, then finish with the remaining green onion and sesame seeds.