This is completely untested, but I just looked at the codex:
http://codex.wordpress.org/Plugin_API/Action_Reference
and I see that there's a hook there called user_register:
http://codex.wordpress.org/Plugin_API/Action_Reference/user_register
... so ... if I had to guess if you try putting this in your functions:
add_action('user_register', 'change_display_name');
function change_display_name($user_id)
{
$args = array();
$args['ID'] = $user_id;
$args['display_name'] = 'Standard Name';
wp_update_user($args);
}
It will give everyone registers the same display name automatically. I assume. However, if you have open registration then every single person who registers will get the same display name.
LoL, edited the code to use the variable $user_id and not $post_id, I'm programmed to use $post_id :P