Basically I created a mu-plugin in wp-content/mu-plugins. Here is a modified version of what I did to go to a custom file:
<?php
/*
Plugin Name: Multisite - Sign-up Page
Version: 1.0
Author: Shawn
*/
/*
* Redirect to correct domain signup (init runs after WordPress has finished loading but before any headers are sent)
*/
function multisite_signup(){
if($_SERVER['REQUEST_URI'] == '/wp-signup.php'){
do_action('signup_header', $_SERVER['HTTP_HOST'].'/my_custom_file.php');
}
}
add_action('init', 'multisite_signup');
/*
* In multisite this runs when the "main domain" signup.php is accessed OR a domain is not found
*/
function signup_redirect($url="#some_default#"){
wp_redirect($url);
exit;
}
add_action('signup_header', 'signup_redirect');