Spray a 13 by 9-inch baking dish with nonstick cooking spray. Tear brioche into 1-inch pieces and place in prepared dish.
In a large bowl, combine milk, custard dessert mix, rum, eggs, and nutmeg. Whisk until well blended. Stir in nuts and raisins, if using. Pour over brioche; press down lightly on brioche to submerge into custard. Let stand for 10 minutes, or until brioche absorbs custard slightly. Bake for 50 minutes, or until top is golden brown and bread pudding puffs. Serve warm with Caramel Rum Sauce.
Caramel Rum Sauce:
Combine caramel sauce, cream, and rum in medium glass bowl. Heat in microwave for 45 seconds. Whisk to blend. Drizzle over bread pudding.