Yes. Please create this directory and file. Adjust as necessary. /wp-content/mu-plugins/s2-hacks.php

<?php
add_filter('site_url', function($url, $path, $scheme){
    if(trim($path, '/') === 'wp-login.php' && $scheme === 'login_post')
        return 'http://mysite.com/my-login-handler.php'; // Custom form action.
    return $url;
}, 10, 3);

See also: Related WordPress source code