You can prepare the roast (through step 2) up to 1 day ahead; cover meat and reserved chili-maple syrup mixture separately, both airtight, and chill. Grill the chilled roast about 35 minutes. From Sunset, AUGUST 2001.
Wearing gloves, remove and discard seeds and veins from chilies. In a blender, whirl chilies, sauce from chilies, maple syrup, and mustard until smooth.
Rinse and dry pork. Trim excess fat and any thin, silvery membrane from tenderloin. Fold thin ends of meat under to make roast uniformly thick and tie cotton string crosswise around each end to secure. Set meat on a rimmed plate; pour about 1/3 cup chipotle-maple syrup mixture over pork and turn to coat completely.
Prepare barbecue for indirect heat according to instructions. Place roast on grill and cook until a thermometer inserted in its center reaches 150 degrees, 20 to 25 minutes.
Transfer roast to a platter and, keeping it warm, let rest 5 minutes. Cut into thin slices and add salt to taste. Serve with remaining chipotle-maple syrup mixture.