Gently mix pork, panko, egg, mixture water, scallions, half the ginger, salt and pepper. Shape 24 equal meatballs and keep refrigerated while heating the sauce.
Bring broth, soy sauce, mirin, brown sugar, garlic and remaining ginger to a simmer in a saucepan. Pour into a 4-quart slow cooker set to HIGH and immediately lower the raw meatballs into the hot liquid with a spoon, arranging them gently in no more than 2 layers.
Cover and cook on HIGH for about 3 hours until the largest meatballs reach 160°F. Avoid stirring for the first 2 hours so the meatballs set before being moved.
Whisk cornstarch with slurry water and fold gently into the sauce. Cover on HIGH for 15 to 20 minutes until glossy and thickened.
During the final half hour, bring rice and rice water to a boil in a saucepan. Cover and cook on low for 13 minutes, quickly scatter broccoli over the rice and cover for 5 minutes more; rest off heat for 10 minutes and fluff gently.
Divide broccoli rice among 6 bowls, add 4 meatballs and sauce to each and divide the measured sesame seeds over the meatballs.