Wash the zucchini well to remove any grit and dry them with paper towels. Trim off the ends and quarter the zucchini lengthwise. Slice off the top 1/4 to 1/2 inch of the soft seed core by running a sharp knife down the length of each quarter; it’s all right if some of the seeds remain. Arrange the zucchini, cut side up, on a baking sheet lined with paper towels. Sprinkle with kosher salt (about 1/2 tsp.) and set aside for 10 minutes. Blot the quarters dry with the paper towels.
Heat a gas grill to medium high or prepare a medium-hot charcoal fire. In a small bowl, mix the cumin, brown sugar, cinnamon, cayenne, 1 tsp. salt, and 1/4 teaspoon pepper. In a medium bowl, gently toss the zucchini quarters with 1 Tbs. olive oil and the spice mix. Let sit for 10 minutes. Thread each onion slice onto two thin skewers (to hold the concentric rings together), brush both sides with 1 Tbs. of the olive oil, and sprinkle with a little salt.
Meanwhile, bring 1-1/2 cups water to a boil in a large saucepan. Stir in the couscous, 2 Tbs. of the olive oil, and 3/4 tsp. salt. Cover, remove from the heat, and set aside.
Set the zucchini cut side down on the grill and cook (if using a gas grill, keep the lid closed), flipping occasionally, until it's browned and softened but not mushy, 6 to 8 minutes. Return the zucchini to its original bowl and toss to pick up any spices clinging to it. Reduce the heat to medium (if using a charcoal grill, put on the lid and partially close air vents). Grill the onions until soft and slightly charred, 8 to 10 minutes. Coarsely chop the zucchini and onions and stir them into the couscous, along with the cilantro, orange zest, and orange juice. Taste for salt and pepper; add a little more orange juice to taste. Serve immediately.
Make a meal of this salad by serving it with grilled shrimp skewers.