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.
To 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, for 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.).