For sauce, in a medium saucepan stir together sugar, cornstarch, and salt; stir in the water and lemon juice. Stir in 2 cups blueberries. Cook over medium heat until thickened and bubbly, stirring frequently. Cook and stir for 2 minutes more. Cool.
For a charcoal or gas grill, place doughnut halves on the rack of a covered grill directly over medium heat. Grill for 2 to 3 minutes or until heated through and golden grill marks appear, turning once halfway through grilling.
Divide doughnut halves among serving dishes. Top with assorted berries, sauce, and whipped topping.