Preheat a grill to medium. Cook the couscous as the label directs. Mix the yogurt, harissa, 1 tablespoon water, and salt to taste.
Slice the pork almost in half lengthwise, stopping about 1/2 inch from the bottom, then open like a book. Cover with plastic wrap and pound with a meat mallet or heavy-bottomed skillet until the pork is about 3/8 inch thick. Remove the plastic. Mix 1 tablespoon olive oil, the garlic, 1/2 teaspoon salt, and pepper to taste, then rub all over the pork.
Toss the zucchini with 1/2 tablespoon olive oil, 1/4 teaspoon salt, and pepper to taste.
Grill, turning once, until lightly charred, about 3 minutes per side. Transfer to a cutting board and chop. Grill the pork, turning once, until just cooked through, about 3 minutes per side. Transfer to a cutting board and let stand 5 minutes.
Toss the couscous, zucchini, tomato, lemon juice, parsley, the remaining 1 1/2 tablespoons olive oil, and salt and pepper to taste. Cut the pork into 4 pieces and serve with the couscous and yogurt sauce.