Stir together slaw mix, parsley, mayonnaise, vinegar, mustard, celery seeds, salt, and pepper in a medium bowl. Set aside.
Preheat a grill to medium-high (400°F to 450°F). Use hands to mix chicken, barbecue sauce, onion, and salt in a bowl. Shape 1/4 cupfuls into 8 small patties, about 2 1/2 in. in diameter and 1/2 in. thick. Place aluminum foil on one side of grill, and coat foil with cooking spray. Place patties on foil, and grill, uncovered, until firm enough to flip, 2 to 3 minutes. Flip, and cook 2 minutes more. Transfer patties to oiled grates, and cook until grill marks appear and a thermometer inserted in center registers 165°F, 1 to 2 minutes per side. During last minute of cooking, add buns to grill, cut sides down; toast lightly.