I have looked around and found that WPMUDev does not offer a "like wordpres.com" domain registeration feature at signup, but I wonder if the following idea could fly:
Add the following options to the sign up page:
At domain selection, user can do one of the following:
1. Register a new subdomain that they would like, which is the normal set up.
2. Input a domain in an "I already have a domain" field, but the client would later map it in admin.
3. "I want to register a new domain" input field - beside it a link to open WHMCS in a new window. - Client would still map it later in admin (or I could).
So, the input fields would not really do anything but notify me that the client has their own domain to be mapped and I could do if for them or email them the instructions. Also, obviously, it the client sets up a sub-domain, nothing changes at all.
If I could set up my sign up page like this, I am sure it would have the professional "like wordpress.com" feel without the need for something like a custom script.
Is this possible?