How do I use the ‘http_request_host_is_external’ filter

I’m having a really hard time trying to use the http_request_host_is_external filter. For some background, I’m trying to set up a separate server to handle private plugin and theme updates. The problem is that it’s on a separate server, so the WordPress wp_http_validate_url (wp-includes/http.php) function kills the request. The following are lines 481-503 of that file.