Hack WordPress to perform an inter-table comparison using 'meta_query'

plugin.php

add_shortcode('bbp-unread-topic-index','dtbaker_bbq_unread_topics');
function dtbaker_bbq_unread_topics(){
    if(is_user_logged_in()){
        $result = '';
        // filter the bbPress query args that are run when [bbp-topic-index] is executed.
        add_filter('bbp_after_has_topics_parse_args','unread_bbp_after_has_topics_parse_args',3,1);
        // adjust the generated 'where' SQL to perform a table comparison
        add_filter('get_meta_sql','dtbaker_get_meta_sql',3,6);
        // run the built in bbpress shortcode which does everything nicely
        $result .= do_shortcode('[bbp-topic-index]');
        // undo our nasty hacks from above.
        remove_filter('bbp_after_has_topics_parse_args','unread_bbp_after_has_topics_parse_args',3,1);
        remove_filter('get_meta_sql','dtbaker_get_meta_sql',3,6);
        // (hopefully) output the list of unread posts to logged in users
        return $result;
    }else{
        return 'Please login to see unread posts';
    }
}
// filter the bbPress query args that are run when [bbp-topic-index] is executed.
function unread_bbp_after_has_topics_parse_args($r){
    $r['meta_query'] = array(
		array(
			'key' => bbp_unread_posts_getLastVisitMetaKey(),
			'value' => '__DTBAKER_SQL_KEY_TO_REPLACE_IN_META_SQL_FILTER__',
			'compare' => '<=',
            'type' => 'SIGNED',
		),
	);
    return $r;
}
// (nasty bit) adjust the generated 'where' SQL to perform a table comparison
function dtbaker_get_meta_sql($sql_array, $queries, $type, $primary_table, $primary_id_column, $context){
    // print_r(func_get_args());
    $meta_table_name = preg_replace('#posts$#','postmeta',$primary_table);
    $sql_array['where'] = str_replace(
        "CAST(mt1.meta_value AS SIGNED) <= '__DTBAKER_SQL_KEY_TO_REPLACE_IN_META_SQL_FILTER__'",
        "CAST(mt1.meta_value AS SIGNED) <= UNIX_TIMESTAMP($meta_table_name.meta_value)",
        $sql_array['where']);
    return $sql_array;
}

http://stackoverflow.com/questions/17416593/wordpress-query-post-compare-between-postmeta-values-using-meta-query-argument