<p>Is it possible, with Javascript or some other technology to determine which hyperlink a user has clicked on, without changing the hyperlink source code.</p>

<p>For example:
Can you click on a 'tag' button, then click on a hyperlink hosted in a different iframe, and be able to calculate which hyperlink the user clicked on, without changing any of the source code in that iframe?</p>

you need to put an event on each a link ,
and then you will get all the information about the specific click.
this will work only in the some document,
so if you try to do a test between the link inside an iframe and a link in your page you will not get an event for the iframe link.
in order to attach the event for all link you need to run on all the links in the page ,
the best way to do that is by jQuery selector. or other js framework like YUI
$("a").click(function () { 
      alert('')
});
getElementsByTagName("a") - will give you all the links in the page.