In a medium bowl, combine the hoisin sauce, pineapple juice, ginger, and sesame oil. Arrange the ribs in a slow cooker and pour half of the sauce mixture over the ribs. Cover and cook on low for 5 to 6 hours or on high for 3 1/2 to 4 hours, until the ribs are very tender. Set the remaining sauce aside in the refrigerator.
About 1/2 hour before the ribs are done, remove the sauce from the refrigerator, to bring it to room temperature.
Arrange the ribs on plates or a platter and brush both sides with some of the remaining sauce. Sprinkle with the scallions and sesame seeds, if using. Serve the rest of the remaining sauce at the table.