In a large dish soak bamboo skewers in warm water to cover for 30 minutes.
Make meatballs: In a large bowl, with your hands, blend the 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.
Make sauce: In a bowl whisk together all sauce ingredients until brown sugar is dissolved. Sauce maybe made 1 day ahead and chilled, covered.
Halve squash lengthwise and cut crosswise into 1/2-inch thick pieces. Quarter onion and then cut the onion and bell peppers into 1-inch pieces.
Assemble kebabs: Thread 2 meatballs alternately with assorted cut vegetables and tomatoes onto each skewer. Kebabs may be assembled 8 hours ahead and chilled, covered.
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 the sauce and grill, 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.