The tokens are not generalised they are also specific to emails.

So the emails look like this :

  'student_course_start'=>array(
            'description'=> __('Student : Student started a course','wplms'),
            'subject' =>  sprintf(__('You started course %s','wplms'),'{{course.name}}'),
            'message' =>  sprintf(__('You\'ve started the course : %s','wplms'),'{{{course.titlelink}}}')
        ),

and the token here is course.titelink. and it is evaluated by our function

function bp_course_wp_mail($to,$subject,$message,$args=''){
    /*=== Migartion to BuddyPRess HTML emails ==*/
    $mails = bp_course_mails::init();
    $y = $mails->emails_migrated(); // for php 5.4 and below
    if( $y && !empty($args['tokens']) ){
     
        $email_type = $args['action'];
        $bpargs = array(
            'tokens' => $args['tokens'],
        );
        bp_send_email( $email_type,$to, $bpargs );
    return;
  }
}

The $args['token'] is a key=>value array 

so course.titlelink is the key and corresponding value is inserted into the mail at the time of processing.

At the time of processing of the email, this token is evaluated.

So now, after recieving your request I am adding this filter :

$bpargs = apply_filters('bp_course_wp_mail_tokens',array(
            'tokens' => $args['tokens'],
        ),$args);

using this filter you can add and evaluate your custom token across all the emails. The $args will help you target one specific mail as well by comparing the $args['action'] with the email action.