Prep: 20 minutes; Marinate: 20 minutes; Cook: 8 minutes.
Whisk together first 6 ingredients. Reserve 1/4 of vinaigrette. Add eggplant, squash, and zucchini. Let stand 20 minutes.
Bring 1 cup water to a boil, and stir in couscous. Remove pan from heat, cover, and let stand 5 minutes. Fluff with a fork; cover and keep warm.
Preheat a lightly oiled grill or grill pan over moderate heat until hot but not smoking. Cook eggplant, squash, zucchini, and onion, turning, 3 minutes per side or until golden brown and tender. Cook tomatoes 30 seconds per side or until grill marks appear. Make 4 stacks, beginning with a slice of eggplant, then 1 tablespoon mozzarella. Continue stacking vegetables, ending with 1 tablespoon mozzarella. Grill stacks (or bake at 350° for 5 minutes), covered, 2 minutes or until cheese melts.
Stir reserved vinaigrette into couscous with chopped basil; spoon onto plates. Top with stacks. Garnish with additional basil, and serve.