Make the pesto: Pulse basil, Parmesan, pine nuts, garlic, salt, and pepper in a food processor until combined. With machine running, add 1/2 cup oil in a slow, steady stream. Transfer to a small airtight container, and top with remaining oil. Pesto can be refrigerated overnight.
Preheat grill or grill pan to medium-high. (If you are using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 3 to 4 seconds.) Grill bell pepper, turning with tongs, until blackened all over. Place in a bag, close bag, and steam 15 minutes. Peel pepper, and discard skin.
Lightly brush both sides of eggplant, zucchini, and summer squash with oil, and season with salt and pepper. Grill, flipping once, until golden brown and soft, 3 to 4 minutes per side.
Pull out most of the doughy center of bread, and reserve for another use (such as breadcrumbs). Brush inside of each loaf with oil. Spread pesto over bottom half of each loaf. Top one loaf bottom with grilled vegetables and half the mozzarella; top remaining loaf bottom with prosciutto and remaining mozzarella. Sandwich top and bottom of each loaf, and press firmly. Tightly wrap each sandwich in plastic, allowing air to release before sealing. Wrap each in parchment paper, and tie with kitchen string. Place sandwiches in refrigerator, and weigh down with a heavy object (such as a Dutch oven). Refrigerate at least 2 hours (or overnight). Slice, and serve.
A mandoline ensures that the vegetables are sliced thinly and evenly. You can use store-bought pesto and jarred roasted red peppers in place of homemade.