Can't get hook_civicrm_contact_get_displayname to work
Can you modify, CRM/Utils/Hook.php

  public static function alterDisplayName($displayName, $contactId, $dao) {


and change it to

  public static function alterDisplayName(&amp;$displayName, $contactId, $dao) {


(note the &amp; character before the displayName argument. I suspect this will fix the issue. 

If it does, can you please file an issue and if you can create a PR to fix this issue. If not, you can reference this thread