Pour 1 1/2 cups buttermilk into a shallow dish. Add the tofu and turn to coat; let marinate at least 10 minutes. Meanwhile, whisk the remaining 1/4 cup buttermilk, the mayonnaise, chives, vinegar, a pinch of salt and a few grinds of pepper in a small bowl.
Put the snap peas in a medium microwave-safe bowl with 1 tablespoon water; cover tightly with plastic wrap. Microwave until just tender and bright green, 1 to 1 1/2 minutes. Discard the plastic wrap and set the peas aside to cool.
Whisk the flour, herbes de Provence and 1/2 teaspoon each salt and pepper in a large bowl. Stir in 2 tablespoons of the buttermilk marinade to make shaggy clumps. Heat 2 inches of vegetable oil in a large pot over medium-heat until a deep-fry thermometer registers 350 degrees F. Dip the tofu in the flour mixture, turning to coat, dip back in the marinade, then redip in the flour to coat with the clumpy bits; transfer to a plate. Working in batches, fry the tofu, flipping occasionally, until golden brown and crisp, 3 to 4 minutes. Remove to a rack and season with salt.
Divide the iceberg wedges among plates; season with salt and pepper. Top with the fried tofu, tomatoes and sugar snap peas. Drizzle with the buttermilk dressing.