Season roast with salt and pepper; place over indirect heat in medium-hot grill. Stir together remaining ingredients and baste roast every 8 to 10 minutes with mixture, until roast is done (internal temperature measured with meat thermometer 155ø-160°F), about 30-45 minutes. Let roast rest for 5-8 minutes before slicing to serve. Discard any leftover basting mixture. Serve on sandwich buns with corn on the cob and celery and carrot sticks.