Preheat your barbecue to medium-high. Brush the roast with olive oil; rub with the chili powder, oregano, cumin, salt and pepper. Set the roast on one side of the barbecue. Turn the heat off underneath it; leave the other side set to medium-high. Close the lid and cook the roast to the desired doneness, about 40 minutes for rare to medium-rare. The best way to gauge doneness is by inserting an instant-read meat thermometer into the centre of the meat, remembering it will continue to cook as it rests after cooking. For a rare roast, the temperature should be 125 degrees F to 130 degrees F; for medium rare it should be 130 degrees F to 135 degrees F; and for medium, it should be 140 degrees F. Set the roast on a plate, tent with foil and allow to rest for 10 minutes.
Make the dipping sauce by combing the remaining ingredients, except buns, in a pot. Gently simmer the sauce for 5 minutes. Thinly slice the beef, pile into the buns and serve with bowls of the sauce for dipping.