Finely shred enough peel from 1 of the limes to make 1 teaspoon; set aside. Juice limes to make 1/4 cup juice. In a small bowl whisk together 1/4 cup lime juice, the olive oil, cumin, salt and pepper.
Cut zucchini lengthwise into 1/2-inch-thick slices. Cut cauliflower crosswise into 4 equal slices. Cut red onion crosswise into 1/2-inch-thick slices. Brush vegetable slices with some of the olive oil mixture.
For a charcoal grill, grill vegetable slices on the rack of an uncovered grill directly over medium coals until crisp-tender, carefully turning vegetables once using a wide spatula. Allow 5 to 6 minutes for zucchini slices and 10 to 12 minutes for cauliflower and onion slices. (For a gas grill, preheat grill. Reduce heat to medium. Place vegetables on grill rack over heat. Cover and grill as above.) Remove vegetables from grill as they get done.
Meanwhile, in a medium saucepan bring water to boiling. Stir in couscous and 1 teaspoon lime peel. Remove from heat; cover and let stand 5 minutes. Fluff couscous mixture with a fork.
To serve, drizzle vegetables and couscous with remaining oil mixture. Top with parsley. Makes 4 servings.