Slice polenta into 1/4-inch-thick rounds. Sprinkle with cinnamon, nutmeg, and brown sugar. Heat butter in a large pan over medium-high heat; add polenta rounds. Sear until golden brown (2-3 minutes on each side). Place frozen blackberries, sugar, and lemon juice in a blender; blend until smooth. Strain the mixture into a bowl, pressing with a spatula. Heat the sauce in a small saucepan over low heat. Cook 3 minutes; add orange-flavored liqueur. Stir until the sauce is heated through. Serve sauce over polenta cakes; top with whipped topping. Sprinkle with orange zest; serve.