Let me write a plugin for this (save as wp-content/plugins/red_visitors.php):
<?php
/**
* Plugin Name: Redirect visitors
* Plugin URI: http://wordpress.stackexchange.com/questions/138519
* Author: still_learning
*/
// function based on http://stackoverflow.com/a/5892694/2948765
// I hope it still works
function __is_login_page() {
return in_array($GLOBALS['pagenow'], array('wp-login.php', 'wp-register.php'));
}
// When WP is ready...
add_action('init', function() {
// Check what kind of page was requested
if(!is_user_logged_in() && !__is_login_page() && !is_admin()) {
// Not logged in, not the login page and not the dashboard
header("Location: http://your-custom-url.com/foo/bar");
exit;
}
});
This script should redirect every visitor to the specified location (I did not test it). It will obviously fail if you destroy the WordPress installation or make severe programming mistakes causing the PHP interpreter to stop interpreting.
I’d not suggest using the same site for development and production, not even if it’s just a redirection. A common approach would be a development server, perhaps accessible over a subdomain (or just a webspace associated with the subdomain), an intra-net server or a local server which can be made available over IPv6.