1. Soak planks in water 1 hour; drain.
2. Preheat grill to medium-high heat.
3. Place broth in a large skillet over medium-high heat; bring to a boil. Add shallots and spinach; cook 5 minutes. Stir in couscous. Remove from heat; cover and let stand 5 minutes. Stir in cheese and next 5 ingredients (through pepper).
4. Cut each zucchini in half lengthwise; scoop out pulp, leaving a 1/4-inch-thick shell. Sprinkle salt evenly over zucchini. Spoon about 2/3 cup stuffing into each zucchini half.
5. Place planks on grill rack; grill 3 minutes or until lightly charred. Turn planks over; place zucchini on charred sides of planks. Cover; grill 12 minutes or until tender.