Heat broiler. Combine sake, 4 tablespoons soy sauce, the broth and sugar in a small saucepan. Bring to a boil and cook until reduced to 1/2 cup.
Mince garlic, grate 1/2 teaspoon yellow zest from lemon. Combine garlic, zest, chicken, ginger, scallion and 1 tablespoon soy sauce.
Shape into 16 balls. Dip each into the sauce. String them onto skewers and put on a rack in a broiler pan. Broil until brown (about 4 minutes/side). Bring sauce back to boil.Serveskewers over rice; pour sauce over meatballs.