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