Preheat the oven to 400 degrees. Core the tomatoes and thinly slice. Place in a colander in the sink and sprinkle with 1/2 teaspoon salt; let drain for at least 15 minutes.
Meanwhile, cook the bacon in a large skillet over medium heat until crisp, about 10 minutes; transfer to a paper-towel-lined plate. Pour off and reserve the drippings, leaving 1 tablespoon in the skillet; toast the bread on one side in the drippings until golden (do this in batches, adding more drippings as needed). Place the bread, toasted-side down, on a baking sheet.
Combine the cheeses with the mayonnaise, scallions and parsley in a bowl. Spread the cheese mixture on the toasts, reserving about 1/3 cup. Crumble the bacon on top. Shake any excess liquid from the tomatoes; distribute the zucchini and tomatoes among the toasts. Dot with the reserved cheese mixture and season with pepper. Bake until golden brown, 15 to 20 minutes. Serve warm.