Place mustard seeds in a mortar. Using a pestle, grind until crushed.
Combine crushed mustard seeds, vinegar, and next 4 ingredients in a small bowl; stir with a whisk. Place pork in a heavy-duty zip-top plastic bag. Pour vinegar mixture over pork; seal. Marinate in refrigerator at least 8 hours, turning bag occasionally.
Remove pork from bag, discarding marinade. Place pork on grill rack coated with cooking spray. Insert meat thermometer into thickest part of pork. Cover and grill 20 minutes or until meat thermometer registers 160°, turning pork occasionally. Remove pork from grill; let stand 10 minutes before slicing.