Step 1Peel and core the pineapple, then cut it into 16 lengthwise spears. Using a spice or clean coffee grinder, grind the long pepper to a medium fine powder. In a small, dry skillet, stir the ground pepper and cinnamon and toast over low heat until fragrant, about 30 seconds. Set aside.
Step 2In a small saucepan over high heat, combine the rum, brown sugar and sweetened condensed milk, stirring constantly until the sugar is completely dissolved and the mixture has come to a boil. Remove the glaze from the heat and stir in the toasted spices.
Step 3Brush the pineapple spears lightly with the glaze, allowing any excess to drip off. Using long-handled metal tongs, place the pineapple spears on a clean, oiled grill over medium heat. Cook for 1 to 3 minutes per side, carefully basting the spears with additional glaze as they cook. (The rum in the glaze can create flare-ups if it drips into hot coals, so use a long-handled brush to apply the glaze and stand back.) The pineapple is done when it is slightly softened and golden brown, with grill marks. Drizzle with more glaze before serving, if desired.