If you set up your permissions correctly, you can still achieve this.
Registered usergroup is the default for everyone. Revoke every kind of permission for this group. So, when a user registers on your site, he won#t be able to do anything, see anything.
He will just see your custom form created by this addon. And after filling the form out, you can then decide to promote him to a usergroup which then puts him to a group what the basic user can do normally.