On a plate, mix together the peppercorns, allspice, cinnamon and salt. Sprinkle the spice rub evenly over the pineapple or roll the pineapple in it so it is evenly coated.
Heat a medium Dutch oven over medium heat. Add the oil and place the pineapple in the pan. Cook, rotating the pineapple regularly, until light brown all over, about 3 minutes. Cover the pan and transfer to the oven. Roast until tender to the core when pierced with the tip of a knife, about 45 minutes. Uncover and allow the pineapple to cool to room temperature in the pan.
Meanwhile, in a medium bowl, combine the cream, sugar and rum. Using a handheld mixer, beat on high speed until soft peaks form, about 3 minutes.
Slice the pineapple into 1-inch-thick rounds. Place 1 slice on each plate. Dollop with a spoonful of cream and a crumble of gingersnaps. Drizzle any remaining juice over the top.