Prepare tomatoes: Heat oven to 300°F. Halve tomatoes through stem ends and arrange, cut sides up, in 1 layer on a large baking sheet. Add garlic to pan and drizzle both tomatoes and garlic lightly with oil (about 1 tablespoon). Sprinkle with salt. Roast in middle of oven until tomatoes are slightly shriveled around edges, about 1 hour. Cool in pan on a rack 20 to 30 minutes.

Meanwhile, make couscous: Bring broth to a boil in a 3-quart heavy saucepan and stir in couscous, then simmer, uncovered, 6 minutes. Cover pan and remove from heat. Let stand 10 minutes. Spread couscous in one layer on a baking sheet or plate and cool 15 minutes.

When tomatoes are done, make dressing: Peel garlic and puree with ¼ cup oil, water, lemon juice, 1 teaspoon salt, ¼ pepper, and ½ cup roasted tomatoes in a blender until dressing is very smooth. Adjust seasonings to taste.

Assemble and serve: Transfer cooled couscous to a bowl and stir in olives, roasted tomatoes, herbs, and dressing. Season to taste with more salt and pepper as needed.

Do ahead: Roasted tomatoes, dressing, and couscous can be made 1 day ahead and kept separately, covered and chilled. Bring to room temperature before proceeding.