1. In a medium bowl, combine squash, broccoli, tomatoes, olives, and parsley; toss with 2 tablespoons salad dressing. Cover and chill for 2 to 24 hours.
2. For hummus: In a food processor, combine cannellini beans, the remaining 2 tablespoons salad dressing, mustard, and salt. Cover and process until smooth and spreadable. (Or mash beans with a potato masher or fork. Stir in salad dressing and mustard.)
3. Cut pita bread rounds in half crosswise; open pita halves to make pockets. Spread hummus inside each pita pocket. Spoon chilled vegetable mixture into pita pockets.