In a large dish soak bamboo skewers in warm water to cover 30 minutes.
In a large bowl with your hands blend together all meatball ingredients until just combined well (do not overmix). Form mixture into 1 1/4-inch balls and arrange on a tray. Meatballs may be prepared up to this point 1 day ahead and chilled, covered.
In a bowl whisk together all sauce ingredients until brown sugar is dissolved. Sauce may be made 1 day ahead and chilled, covered.
Halve squash lengthwise and cut crosswise into 1/2-inch-thick pieces. Quarter onion and cut onion and bell peppers into 1-inch pieces.
Thread 2 meatballs alternately with assorted cut vegetables and tomatoes onto each skewer. Kebabs may be assembled 8 hours ahead and chilled, covered. Prepare grill.
Grill kebabs on an oiled rack set 5 to 6 inches over glowing coals, turning kebabs frequently, 10 minutes. Brush kebabs with sauce and grill, turning them and brushing frequently with sauce, until meatballs are cooked through, about 5 to 10 minutes more. (Alternatively, kebabs may be broiled under a preheated broiler about 3 inches from heat.) Discard any remaining sauce.
