Preheat a grill to medium. Cook the couscous as the label directs. Mix the yogurt, hot sauce, 1 tablespoon water, and salt to taste in a small bowl.
Butterfly the pork: Slice almost in half lengthwise, stopping about ½ inch from the other side, then open like a book. Cover with plastic wrap and pound with a meat mallet or heavy-bottomed skillet until the pork is a little less than ½ inch thick. Remove the plastic. Mix 1 tablespoon olive oil, the garlic, ½ teaspoon salt, and pepper to taste, then rub all over the pork.
Toss the zucchini with ½ tablespoon olive oil, ¼ 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 rest 5 minutes.
Toss the couscous, zucchini, tomato, lemon juice, parsley and the remaining 1½ tablespoons olive oil in a bowl; season with salt and pepper. Cut the pork into 4 pieces and serve with the couscous and yogurt sauce.
