To make the rub: Combine all ingredients except the roast, in the bowl of a food processor, spice grinder, or mortar and pestle. Grind until the nibs break into particles the size of large grains of sand. Use immediately or store in a tightly covered jar for up to 1 month.
Rinse and pat the roast dry with paper towels. Generously cover the meat with the rub and wrap in foil or plastic wrap. The roast can marinate overnight in the refrigerator.
Remove the roast from the refrigerator and bring to room temperature. In the meantime, fire up the grill. Prepare the grill for indirect cooking, with the coals to either side. Place the meat on the grill, fat side up, not directly over the coals. The internal grill temperature should be between 275 and 300 degrees F. Grill the meat for 40 minutes. Check the internal temperature with an instant-read thermometer. For medium rare meat, remove from the grill when the thermometer registers 125 degrees F. Let the roast sit, loosely covered with foil, for several minutes before slicing.