To make coulis: Trim ends of 2 zucchini and cut into 1/2-inch chunks. In a medium saucepan, place zucchini, 1 cup water, and 1/2 teaspoon salt. Bring to a boil; lower heat to medium and cover pan. Cook until zucchini is very soft, about 15 minutes. Transfer cooked zucchini and liquid to a blender and process until very smooth. Add a little water if too thick. Stir in 1 tablespoon rosemary oil and freshly ground black pepper. Cool to room temperature.
Meanwhile, trim ends of remaining 6 zucchini and cut each in half lengthwise. Place on a large rimmed baking sheet. Drizzle 2 tablespoons rosemary oil over zucchini and roll zucchini around to coat thoroughly. Sprinkle lightly with salt. Place cut side up on baking sheet and roast until soft, about 15 minutes. Transfer to a platter, cut side up; let cool. Drizzle both sauces over zucchini in a striped pattern.