Generously grease a 2-quart glass or ceramic heat-proof bowl with nonstick cooking spray. Roll up a piece of aluminum foil to form a ring and place in the bottom of a large slow cooker.
In a large bowl mix together the cereal, apple, flour, sugar, 1 teaspoon pumpkin pie spice and 1/2 teaspoon salt and stir to combine.
In another bowl, whisk together the eggs, milk, butter, 1/4 cup spiced rum, and vanilla.
Make a well in center of the dry ingredients, add the liquid ingredients then stir to combine. Put the batter into the prepared bowl, and smooth top with spatula. Cover with aluminum foil and secure with a large elastic band so water does not seep inside. Place the bowl on the foil ring placed inside the slow cooker and fill with water to come 2/3 the way up the side of the bowl. Cook on high for 4 hours or until cake is set in the center. Remove the cake from the slow cooker. When cool enough to handle invert onto a plate to unmold.
Combine the whipped topping, remaining 2 tablespoons spiced rum and remaining 1/2 teaspoon pumpkin pie spice and mix until well incorporated.
Serve the cake warm with a dollop of spiced whipped topping.