Put oven racks in upper and lower thirds of oven and preheat oven to 450°F. Halve squash lengthwise, then cut off and discard stem ends. Scoop out seeds and cut squash lengthwise into 3/4-inch-wide wedges. Toss squash with black pepper, 3/4 teaspoon salt, and 2 tablespoons oil in a bowl, then arrange, cut sides down, in 2 large shallow baking pans.
Roast squash, switching position of pans halfway through roasting, until squash is tender and undersides of wedges are golden brown, 25 to 35 minutes.
While squash roasts, mince garlic and mash to a paste with remaining 1/4 teaspoon salt. Transfer paste to a small bowl and whisk in lime juice, hot pepper (to taste), cilantro, and remaining 1/4 cup oil until combined.
Transfer squash, browned sides up, to a platter and drizzle with vinaigrette