Gently mix pork, panko, egg, mixture water, scallions, half the ginger, salt, and pepper. Shape 24 equal meatballs; refrigerate while heating sauce.
Simmer broth, soy sauce, mirin, brown sugar, garlic, and remaining ginger in a saucepan. Pour into a 4-quart slow cooker on HIGH; immediately lower raw meatballs into hot sauce with a spoon, gently arranging in no more than 2 layers.
Cover; cook on HIGH about 3 hours until largest meatballs reach 160°F. Do not stir for first 2 hours so they set.
Whisk cornstarch with slurry water; gently fold into sauce. Cover; cook on HIGH 15 to 20 minutes until glossy and thickened.
During final half hour, boil rice and rice water in a saucepan. Cover; cook on low 13 minutes. Quickly add broccoli on top; cover 5 minutes more. Rest off heat 10 minutes; fluff gently.
Divide broccoli rice among 6 bowls; add 4 meatballs and sauce to each. Divide measured sesame seeds over meatballs.