Captcha free blocking of spam - uses JavaScript/AJAX

Just came across this interesting approach to captcha free blocking of spam by Saurabh Gupta.
Wondered if something like this could be applied to PunBB as extension?

[code=php]
<?php
/*
Plugin Name: WP Captcha Free
Plugin URI: http://wordpresssupplies.com/wordpress-plugins/captcha-free/
Description: Block comment spam without captcha.
Author: iDope
Version: 0.7
Author URI: http://wordpresssupplies.com/
*/

/*  Copyright 2008  Saurabh Gupta  (email : saurabh0@gmail.com)

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

// are we inside wp?
if(!defined('ABSPATH')) {
    // check if this is an ajax post
    if(isset($_POST['post_id'])) {
        // find wp-config.php
        if(file_exists('../../wp-config.php')) {
            $includefile='../../wp-config.php';
        } else if(file_exists('../../../wp-config.php')) {
            $includefile='../../../wp-config.php';
        } else {
            die('alert("Unable to include wp-config.php. Please make sure \'captcha-free.php\' is uploaded to the \'wp-content/plugins/\' folder.")');
        }
        // load wordpress
        require_once($includefile);
        nocache_headers();
        $post_id = intval($_POST['post_id']);
        $timehash=timehash($post_id,time());
        echo "gothash('$timehash')";
    }
    exit;
}

// generate random salt on activation
register_activation_hook(__FILE__,'cf_make_salt');
function cf_make_salt() {
    update_option('cf_salt',mt_rand());
}

add_action('init', 'cf_init');
function cf_init() {
    // Bypass check for logged in users (except 'subscriber')
    if(!current_user_can('level_1')) {
        add_action('wp_head', 'cf_js_header' );
        add_action('comment_form', 'cf_comment_form', 10);
        add_action('preprocess_comment', 'cf_comment_post');
    }
}

// add javascripts
function cf_js_header() {
    wp_print_scripts( array( 'sack' ));
}

// add hidden field for hash and ajax stuff to the form
function cf_comment_form($post_id) {
    ?>
<script type="text/javascript">
//<![CDATA[
    function gethash(){
        document.getElementById('commentform').onsubmit = null;
        if(document.getElementById('submit')) document.getElementById('submit').value='Please wait...';
        var mysack = new sack("<?php echo get_option('siteurl').cf_get_path().'captcha-free.php'; ?>");
        mysack.execute = 1;
        mysack.method = 'POST';
        mysack.onError = function() { alert('Unable to get Captcha-Free Hash!') };
        mysack.setVar('post_id', <?php echo $post_id; ?>);
        mysack.runAJAX();
        return false;
    }
    function gothash(myhash){
        document.getElementById('captchafree').value = myhash;
        // Workaround for Wordpress' retarded choice of naming the submit button same as a JS function name >:-(
        document.getElementById('submit').click();
    }
    document.getElementById('commentform').onsubmit = gethash;
//]]>
</script>
<input type="hidden" id="captchafree" name="captchafree" value="" />
<p><small><noscript><strong>Please note:</strong> JavaScript is required to post comments.</noscript> <a href="http://wordpresssupplies.com/wordpress-plugins/captcha-free/">Spam protection by WP Captcha-Free</a></small></p>
<?php
}

// Validate the hash
function cf_comment_post($commentdata) {
    // Ignore trackbacks
    if($commentdata['comment_type']!='trackback') {
        // Calculate the timehash that is valid now
        $timehash=timehash($commentdata['comment_post_ID'],time());
        // Calculate the timehash that was valid 1 hour back to give some cushion
        $timehash_old=timehash($commentdata['comment_post_ID'],time()-3600);
        if($_POST['captchafree']!=$timehash && $_POST['captchafree']!=$timehash_old)
            wp_die('Invalid Data: Please go back and try again.');
    }
    return $commentdata;
}

// generate a hash for a given post and timestamp
function timehash ($post_id,$timestamp) {
    // Make a hash out of stuff that shouldn't change between requests
    return md5(get_option('cf_salt').$post_id.date('yzH',$timestamp).$_SERVER['REMOTE_ADDR'].$_SERVER['HTTP_USER_AGENT']);
}

// Get virtual path to this plugin
function cf_get_path() {
    $rootpath = preg_replace('|\\\\+|','/',ABSPATH); // Cater for Windows paths
    $rootpath = untrailingslashit($rootpath); // Remove trailing slash if exists
    $mypath = preg_replace('|\\\\+|','/',dirname(__FILE__));
    $mypath = str_replace($rootpath,'',$mypath); // just get the virtual path
    $mypath = trailingslashit($mypath); // Add trailing slash
    return $mypath;
}
?>
[/code]