Pierce potatoes several times with a fork and place on a microwave-safe plate. Microwave on high for 15-20 minutes or until tender, rotating the potatoes once. Let stand until cool enough to handle.
Meanwhile, in a large resealable plastic bag, combine the summer squash, zucchini, peppers and onion. Pour salad dressing over vegetables. Seal bag and turn to coat; marinate for 20 minutes.
Cut each potato in half lengthwise. Scoop out pulp, leaving a thin shell (save pulp for another use). Brush inside of shells with oil and sprinkle with 1/4 teaspoon salt.
Coat grill rack with nonstick cooking spray. Place potato shells skin side up on grill rack. Grill, covered, over indirect medium heat for 10 minutes or until golden brown.
Drain vegetables, reserving marinade. Grill vegetables in a grill basket, uncovered, over medium heat for 10 minutes or until tender, basting with reserved marinade.
Sprinkle potato skins with cheese. Fill with grilled vegetables; sprinkle with remaining salt. Grill 5 minutes longer or until cheese is melted.