If you missed this article (in french) on bioinfo-fr, I will just explain the code, it could be of some help.
The code here is very simple :
location.hostname=location.hostname+'.urlproxy'; The interesting part : It adds at the end of the domain name (location.hostname) the proxy’s URL, wich automatically change the URL of the page on wich you activate the bookmarklet.
If you get a 404 error. It means that the proxy was not planned for the visited site. Wich could happen if you click twice on the bookmarklet.