Heat the oven to 425°F and oil a rimmed sheet. Soak panko in milk for 3 minutes, then gently mix with chicken, squeezed zucchini, egg, onion powder, celery seed, salt and pepper. Form 24 equal meatballs.
Bake for 16 to 20 minutes until the centers reach 165°F. Meanwhile, warm hot sauce, broth, 3 tablespoons butter and brown sugar in a saucepan until the butter melts and the sauce steams.
Transfer hot sauce and hot meatballs to a 4-quart slow cooker. Cover and cook on HIGH for 2 hours, turning gently through the sauce once.
Whisk cornstarch with slurry water and stir into the sauce. Cover on HIGH for 15 minutes until lightly thickened, then turn off the cooker and stir in remaining butter; verify the meatballs remain at least 165°F.
Spoon into a serving bowl and scatter the measured blue cheese over the meatballs. Serve about 3 meatballs per appetizer portion while hot.