If you want to customize system links without editing the HTML templates directly, you can do that by editing the /application/config/custom_redirects.php file.
Set the custom_system_url_redirects array to have the original link and the redirect link. For example:
$config['custom_system_url_redirects'] = array('register' => 'cart/add/8');
This will point the 'register' URI to go to 'cart/add/8' at all times.
