In a medium saucepan, bring the 2-1/4 cups water to boiling. Meanwhile, in a medium bowl, combine cornmeal, the 3/4 cup cold water, the brown sugar, salt, and allspice. Slowly add cornmeal mixture to boiling water, stirring constantly. Cook and stir until mixture returns to boiling. Reduce heat to low. Cover and cook for 10 to 15 minutes or until mixture is thick, stirring occasionally. Stir in orange peel. Pour hot mixture into a buttered 882-inch baking pan; cool. Cover and chill for at least 4 hours or until firm.
Remove polenta from pan. Cut into 4-inch squares. Brush both sides of polenta squares with melted butter.
For a charcoal grill, place polenta squares on the rack of an uncovered grill directly over medium coals. Grill for 8 to 10 minutes or until lightly browned, turning once halfway through grilling. Add plums, cut sides down, to the grill during the last 4 to 5 minutes of grilling or until tender and heated through, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place polenta squares, then plums on grill rack over heat. Cover and grill as above.)
To serve, cut each polenta square in half diagonally, forming eight triangles. Arrange a polenta triangle and two plum halves on each dessert plate. Drizzle plums with balsamic vinegar. Sift powdered sugar over each serving and, if desired, garnish with thyme sprigs. Makes 8 servings.