Automated contact filtering for event attendees?
You can write a plugin that hooks into civicrm_post that will allow you to do anything you like with the newly-registered contact.

add_action( 'civicrm_post', 'my_do_stuff_after_registration', 10, 4 );
function my_do_stuff_after_registration( $op, $objectName, $objectId, $objectRef ) {
    if ( $op != 'create' ) return;
    if ( $objectName != 'Participant' ) return;
    $contact_id = $objectRef['contact_id'];
    // ... do anything you like with this contact
}