Prepopulate registration fields

Hello,

I have installed the prepopulate module and I can make it work with the login page using ?edit[name]=test but that syntax will not work with the user registration. All of the fields in the registration form are within s, most of which are not named. I have installed regcode and would like to simplify registration by embedding the code in the invitation email but I cannot find a way to do so. I have also installed CAPTCHA/reCAPTCHA, though I don't think that'd cause any problems. How would I go about getting this done?

Thanks