CakePHP 2.X Order on Field in HABTM Join Table

Question

I have these 3 tables:

                      cols
groups           | id | ... | ...
groups_users     | group_id | user_id | created
users            | id | ... | ...

In my Group model, I have a HABTM relation with the User model (using the alias Member).

public $hasAndBelongsToMany = array(
    'Member' => array(
       'className'              => 'User',
       'joinTable'              => 'groups_users',
       'foreignKey'             => 'group_id',
       'associationForeignKey'  => 'user_id',
       'unique'                 => true,
   ),
);

This works well if I'm just trying to get all of the members of a group and even order by fields in the users table such as their id or username. However, I have a situation where I need to order the results by the join table. I would like to display the members that have joined the group most recently, so 'groups_users.created' => 'DESC'

I've tried adding the following lines to the HABTM array but it didn't work

'with'   => 'GroupsUser',
'order'  => array('GroupsUser.id' => 'DESC')

I also created a model for GroupsUser like so but this also did not work

class GroupsUser extends AppModel {
    public $name = "GroupsUser";
    var $useTable = "groups_users";

    public $order = array(
        'GroupsUser.created' => 'DESC',
        'GroupsUser.id' => 'DESC'
    );

The following DOES WORK but I want to get away from calling the GroupsUser model, I would like to stick to using the HABTM Member relation.

$this->Group->GroupsUser->find('all',
    array(
        'limit' => 20,
        'order' => array('GroupsUser.created' => 'DESC'),
        'conditions' => array(
            'GroupsUser.group_id' => $group_id,
            'not' => array(
                'OR' => array(
                    array('User.username' => NULL),
                    array('User.username' => 0),
                    array('User.username' => '')
                )
            )
        ),
        'contain' => array(
           'User' => array(
                'fields' => array('User.username'),
            ),
        )
    )
);