I need help from a prototype.js expert:

I'm trying to achieve the following (taken from the addthis forum), and port the solution from jquery to prototype.js (I'm using magento).

Original post is here: http://www.addthis.com/forum/viewtopic.php?f=3&t=22217

For the getScript() function, I can create a custom function to load the remote js, however I'm trying to load the js file via the prototype ajax call, and trying to avoid having the script cached in the browser.

<script type="text/javascript" src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript">

$(document).ready(function() {

  $("#changeURL").click(function() {
    $(".addthis_button").attr("addthis:url","http://www.example.com");
       window.addthis.ost = 0;
       window.addthis.ready();
  });
});

// prevent jQuery from appending cache busting string to the end of the URL
var cache = jQuery.ajaxSettings.cache;
jQuery.ajaxSettings.cache = true;
jQuery.getScript('http://s7.addthis.com/js/250/addthis_widget.js');

// Restore jQuery caching setting
jQuery.ajaxSettings.cache = cache;

</script>

<p id="changeURL">Change URL</p>
<a class="addthis_button" addthis:url="http://www.google.com"></a>
<script type="text/javascript" src="http://s7.addthis.com/js/250/addthis_widget.js#username=rahf"></script>