Add moved signup URL to menu item. Pro sites. Anti Splog

I am using Anti Splog to move my wp-signup.php url to signup-xxx. What I want to achieve is adding this changing url to my menu.

I have tried using the shortcodes in menu plugin as advised on another forum but it doesn't work for me - it just returns http://www.mysite.com/ust_wpsignup_url as the address

I am currently using this code in my themes functions.php file as advised on another forum

add_filter( 'wp_nav_menu_items', 'add_loginout_link', 10, 2 );
function add_loginout_link( $items, $args ) {
    if (is_user_logged_in() && $args->theme_location == 'primary') {
        $items .= '<li><a href="'. wp_logout_url() .'">Log Out</a></li>';
    }
    elseif (!is_user_logged_in() && $args->theme_location == 'primary') {
        $items .= '<li><a href="'. site_url('wp-login.php') .'">Log In</a></li>';
    }

	$signup_url = do_shortcode("[ust_wpsignup_url]");
        $items .= '<li><a href="'. $signup_url .'">Sign Up</a></li>';
	return $items;
}

The above code also adds a login/logout menu item just FYI. This however returns http://www.mysite.com/ust_wpsignup_url as the link address.

I have also tried a slight variation on the code above where I don't use the shortcode but the php hook itself (Sam code as above but with this replacement):

$items .= '<li><a href="'. ust_wpsignup_url() .'">Sign Up</a></li>';

And this displays the actual link as text in the nav bar, and the nav url just links to the current page.

This seems like it should be a simple thing to do - can someone please advise on how to make this work