modifying registration form & blog creation

Hi, I'm currently using the 2.1 beta 2 version of the Membership plugin

A couple of questions:

1. Is it possible to make changes to the registration form without hacking the core file? (registration.form.php) Perhaps similar to the way child themes work. I would like to make some changes but don't want to have them overwritten with future changes.

2. I have a bit of a dilemma regarding blog/site creation. If "Logged in users may register new sites" is turned OFF on my multisite installation settings, then no user can create a new blog. I do want however, certain users to be able to create sites. So this must stay on for now.

Under the membership plugin options, I also want “Enable incomplete signup accounts” ON. This is so that after a user creates an account, but accidentally navigates away from the form for any reason, they just need to login to the site visit the subscription page, and select a subscription. If “enable incomplete signup accounts” is turned off, and they don’t choose a plan for whatever reason, come back later, they are locked out and will get the “account is not active message” on the wp login screen. Not only that, but they can’t sign up again using their email address on the registration form because their email is already in use. So this must also stay ON, or we might lose potential members.

The downfall to having the combination of "logged in users may register new sites" and "enable incomplete signup accounts" is that now any user can simply sign up, NOT select a subscription and go ahead and sign up for a site/blog. Not exactly ideal, as the membership level should determine who can get a site. For example on a "pro" level, I have an associated role, and I want JUST that WP role to be able to create a new. I'm using the "blog creation" rule in the positive area to achieve this.

Am I overlooking something? Could the membership plugin somehow determine the site creation, rather than WP?