Arrange wings in a 6-quart slow cooker. Whisk soy sauce, mirin, brown sugar, ginger, garlic and rice vinegar and pour over the wings.
Cover and cook on low for 3 to 4 hours until the thickest meat reaches 165°F and the wings are tender but still hold together.
Heat the broiler and oil a foil-lined rimmed sheet pan. Transfer wings to the pan, then skim fat from the cooking liquid and pour it into a saucepan.
Simmer sauce for 5 minutes. Mix cornstarch with cold water, whisk into the simmering sauce and cook for 1 to 2 minutes until it coats a spoon.
Brush wings with thickened sauce and broil 6 inches from the heat for 3 to 4 minutes, turn and brush again, then broil for 2 to 3 minutes until lightly browned.
Divide among 6 appetizer servings, spoon over any remaining glaze and add the measured sliced scallions evenly.