In device&user mode, a user that is not logged should be displayed grayed out. Adhoc devices have a special greenish color. So, the feature is there. Be sure that you enable "all" permissions in /etc/asterisk/manager.conf, if user events are filtered, several status involving device&user mode will not work. Normally the "user" permission is fine, but trixbox is broken and requires "all" , I am not totally sure if this is a problem with some particular asterisk version, or only trixbox. ( but I have seen this only on trixbox setups).

Best regards,