Weird one but is it possible to make <a href="http://www.helloworld.com">Link</a>
work without the text helloworld.com being in the source?
To what end? If you want to totally prevent users from figuring out the URL before they click the link, no that's impossible. If instead you just want to prevent that URL from being picked up by scrapers that don't run JavaScript, then yes that is entirely possible.
What I would do in that case is add a data attribute to the link with an encoded version of the URL.
<a href="#" data-href-rot13="uggc://jjj.uryybjbeyq.pbz">Link</a>
Then in your JavaScript, decode. Untested jQuery example to get you started:
$('a[data-href-rot13]').each(function (index, el) {
$(this).attr('href', rot13($(this).attr('data-href-rot13')));
});
Of course, you might want a different algorithm than rot13. You could use AES and base-64 encode the output for use in the tag. In case you do want rot13, see this post: https://stackoverflow.com/a/15747894/362536