To prepare sauce, combine the first 10 ingredients in a small bowl, stirring with a whisk.
To prepare the tofu, cut tofu lengthwise into 3 slices. Cut the tofu slices in half vertically. Place the tofu slices on several layers of heavy-duty paper towels; cover with additional paper towels. Let stand for 30 minutes, pressing down occasionally.
Combine 1 tablespoon soy sauce, oil, and 1/4 teaspoon salt in a small bowl. Brush the tofu slices with soy sauce mixture. Place the tofu slices and bell pepper on grill rack coated with cooking spray, and grill 4 minutes on each side or until tofu is golden.
Combine sauce, bell pepper, watercress, and cucumber in a large bowl, tossing gently to coat. Fill each pita half with about 1/2 cup watercress mixture and 1 tofu slice.