* Gets the IDs of threads that the specified user has not read. Doesn't not work for guests.
* Doesn't include deleted.
* @param integer $userId
* @param array $fetchOptions Fetching options; limit only
* @return array List of thread IDs
public function getUnreadThreadIds($userId, array $fetchOptions = array())
XenForo_Model_Thread, line 450. You should be able to run that and return the results using the $viewParams inside your extended ControllerPublic.