In a zip-lock plastic bag, mix shallots, fish sauce, brown sugar, and pepper. Rinse pork butt. Rinse pork. Slice into thin strips (1/8 in. thick; if pork is difficult to slice thinly, freeze just until firm, 20 to 30 minutes) and add to bag. Seal bag, turn to coat meat, and chill for 1 hour. Thread pork strips flat onto wooden skewers (cut blunt ends off skewers if necessary to fit in pan). Discard marinade.
Set a 10- to 12-inch ridged grill pan over high heat. When hot, lightly coat with vegetable oil. Working in batches, lay pork skewers flat in pan (do not crowd). Cook, turning once, until browned on both sides and no longer pink in the center (cut to test), 6 to 8 minutes total. Transfer skewers to a platter or small plates and serve with Thai pummelo salad on the side.