New Blog Template with custom registration form?

I am using WPMU with Pro Sites and a custom Gravity Forms registration page. I have tested New Blog Templates using the default wp-signup.php and I would like to add that functionality to my custom registration. I have already modified the Gravity Forms Registration add-on code to work with Multi Domain. Does anyone know which hooks New Blog Templates is using to add the template selection option to wp-signup.php?

Thanks,
Brian