If you want to include a custom unsubscribe url, i think the correct way would be so that mailwizz would add a hook for this, and then you could, from an extension, do something like:
PHP:
Yii::app()->hooks->addFilter('campaign_common_tags_search_replace', function(array $tags, Campaign $campaign, ListSubscriber $subscriber = null){
$tags['[UNSUBSCRIBE_URL]'] = 'http://you-customer-unsubscribe.url/?subscriber_email=' . $subscriber->email;
return $tags;
});
But even so, if you send the subscriber to unsubscribe on an external url, how will you notify mailwizz that the subscriber unsubscribed?