I am trying to produce a reference registration system in Wordpress. The concept would be that the user will click a hyperlink that will contain wordpress-login.php?action=register&ref=12345

I am connecting in to the 'register_form' action hook and adding a reference number hidden input as well as on registration use that reference number to complete whatever i want related to it.

The problem seems once the user will get a problem within the registration form. However , he'll be come to wherever the experience attribute from the register form states that is "wordpress-login.php?action=register" with no ref parameter. I am searching for a method to modify that action attribute. I have looked within the wordpress-login.php file also it appears that there are no filter there.

I understand almost always there is javascript, but can there be another wordpressy method of doing this?

I think you will get a lot of clues out of this affiliate marketers wordpress plugin, you are able to download it and checkout the code from http://wordpress.org/extend/plugins/affiliates/

Best of luck,