Preheat the oven to 375°. Spread the tomatoes on a large rimmed baking sheet. Drizzle with 1/4 cup of the olive oil and sprinkle with the salt. Scatter the thyme sprigs over the tomatoes and roast in the oven for 1 hour, until the tomatoes are softened and beginning to brown. Discard the thyme sprigs. Transfer the tomatoes and their juices to a bowl.
Preheat the broiler. Score the flesh sides of the eggplant halves in a diamond pattern about 1/2 inch deep. Season the scored halves with salt and pepper and rub with olive oil. Arrange the eggplant halves on 2 baking sheets, skin side up. Broil the eggplant about 6 inches from the heat for about 20 minutes, until their skins are charred and the halves have collapsed. Use a spatula to gently loosen the eggplants from the baking sheets. Let cool for 20 minutes.
Meanwhile, heat 1 tablespoon of the olive oil in each of 2 large, deep skillets. Add 2 cups of the couscous to each skillet and cook over moderate heat, stirring constantly, until the couscous is deep golden, about 5 minutes. Add 2 cups of the water to each skillet and bring to a boil. Cook until the couscous is barely tender, about 5 minutes. Remove the skillets from the heat and cover. Let stand until all of the water has been absorbed, about 30 minutes.
Transfer the couscous to 2 large bowls. Scoop the roasted eggplant flesh onto a cutting board and chop coarsely. Add the eggplant and the roasted tomatoes to the couscous along with the minced preserved lemon and the parsley; season with salt and pepper and toss gently but thoroughly. Transfer the couscous to large platters and serve.
The couscous salad can be prepared up to 2 hours ahead.