Heat the oven to 425°F and oil a rimmed sheet. Soak panko in milk for 3 minutes, then mix gently with chicken, egg, Parmesan, meatball salt and pepper. Shape 24 meatballs and bake for 16 to 20 minutes until the centers reach 165°F.
Meanwhile, simmer broth, wine and garlic in a saucepan for 3 minutes. Transfer hot liquid and hot meatballs to a 4-quart slow cooker.
Cover and cook on HIGH for 2 hours, turning gently once. Whisk cornstarch with slurry water, fold into the sauce and cover on HIGH for 15 minutes until lightly thickened.
Turn the cooker off, wait until bubbling settles and stir in lemon juice, drained capers and 2 tablespoons butter. Confirm the meatballs remain at least 165°F.
During the final 15 minutes, boil orzo in a large pot of water seasoned with orzo-water salt for 8 to 10 minutes, or the package time, until tender. Drain and toss with remaining butter.
Divide orzo among 6 bowls and top each with 4 meatballs and lemon-caper sauce. Serve immediately.