How do I restrict the domains where users can open up a blog?

I want to restrict the sites a user can sign up to...

I am aware of the anti-splog and sign up code plugins, but I need something a bit more custom tailored...

When I try to start a new blog at wp-signup.php, there is a dropdown list showing all the domains in my network. I want to show only one or few domains based on a user's profile field. How can I do that?