Grill 5-6 minutes per side, brushing frequently with half of the barbecue sauce.
Remove tenderloin from the grill and glaze with a thick coat of remaining barbecue sauce.
Cover pork and let rest for 10 minutes.
Slice into 1/2-inch thick medallions. Serve warm.
For pineapple relish: Combine all the remaining ingredients in a medium bowl. Toss, cover, and refrigerate.
To assemble sandwiches: Bring pineapple relish to room temp before assembling. Slice open rolls almost all the way and warm on a low-heat grill or an oven. Place a slice of pork in roll. Top pork with a spoonful of relish.