1. Wearing gloves, remove and discard seeds and veins from chilies. In a blender, whirl chilies, sauce from chilies, maple syrup, and mustard until smooth.
2. 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.
3. Prepare barbecue for indirect heat.
If using charcoal briquets, mound and ignite 60 briquets on the firegrate of a barbecue with a lid. When briquets are dotted with gray ash, in 15 to 20 minutes, push equal amounts to opposite sides of firegrate. Add 5 more briquets to each mound of coals now and after 30 minutes, if grilling takes longer than that. Prepare barbecue for indirect heat.
If using a gas barbecue, turn all the burners to high, close lid, and heat for 10 minutes. Then adjust burners for indirect cooking (heat on sides of grill not down center under food) and keep on high.
Set a drip pan on firegrate between coals or burners. Set barbecue grill in place. Set roast in center of grill, not over heat. Cover barbecue; open vents for charcoal.
Place roast on grill and cook until a thermometer inserted in its center reaches 150°, 20 to 25 minutes.
4. 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.