Adjust one oven rack to middle position and a second rack 6 inches from broiler element and heat oven to 425°F Grate Parmesan (1 1/2 cups). Mince shallots. Mince 3 garlic cloves. Combine orzo, oregano, pepper flakes, 1/2 cup grated Parmesan, shallots, minced garlic and 1/4 teaspoon salt in bowl. Spread mixture evenly into broiler-safe 13-by-9-inch baking dish.
Slice zucchini and squash 1/4 inch thick. Core tomatoes and slice 1/4 inch thick. Alternately shingle zucchini, squash and tomatoes in tidy rows on top of orzo.
Carefully pour broth over top of vegetables. Bake on lower rack until orzo is just tender and most of broth is absorbed, about 20 minutes. Meanwhile, slice four 3/4-inch-thick slices of bread and spread out evenly over rimmed baking sheet; set aside.
Remove dish from oven and heat broiler. Drizzle vegetables with 1 tablespoon oil, and sprinkle with remaining 1 cup Parmesan. Broil on lower rack until nicely browned and bubbling around edges, about 5 minutes. Remove dish from oven and let rest for 10 minutes.
While tian rests, broil bread on upper rack, flipping as needed, until well toasted on both sides, about 4 minutes. Peel remaining 1 garlic clove and rub one side of each toast with garlic, then drizzle toasts with remaining 1 tablespoon oil and season with salt and pepper to taste.
Chop 2 tablespoons basil and sprinkle over tian. Serve with toasts.