Quickest Way To Create a Pre-Register Form
What you suggest (using Rules for this) is far too complicated for me.

To do this with the minimum hassle, I would just set up a standard Drupal site and create a fixed front-page that says: "Pre-register for my great new site."   Configure permissions so that if the logged in user doesn't have admin permissions, all he/she is allowed to see is this fixed front page.

Then use String Overrides to replace the text "Create new account" in the login box with the text: "Click here to pre-register".  I.e. the by "pre-registering", the user really creates a standard account on the site.

Then auto-subscribe everyone that pre-registers to a newsletter (for a newsletter module you can, for example, use  Notify). This will let you post updates on the site about the ongoing development, etc. that also are automatically sent to those who've pregistered as a digest newsletter e-mail.  (Notify let you auto-subscribe new users and also has a master switch to make subscribing and unsubscribing a breeze).

Later, when you've done setting up the site, you can ask the user to confirm his/her committment by ticking some field in the user profile, and silently convert those that confirm to regular users by given additional access permissions (and delete the rest).

Disclosure: I am the maintainer of Notify.