Plug-in to change target="_blank" to rel="external" to fix validation?

// Get the whole page, right before it's rendered to the browser
$source = &$modx->documentOutput;

// Syntax to find and replace
$strFind = 'target="_blank"';
$strRepl = 'rel="external"';

// Do the thing that finds and replaces stuff
$source = str_replace($strFind, $strRepl, $source);

jQuery.noConflict();
jQuery(document).ready(function($){
	// Open external links in a new window
	$('a[href^=http]').each(function(){
		if(this.href.indexOf(location.hostname) == -1) {
			$(this).attr('target', '_blank');
		}
	});
});