Domain Mapping By Default During Registration

I have set up my server to act as a rapid deployment WordPress site for some of my "lower value" clients.

What I'd love to be able to do is let them pick their already registered FQDN during the registration process instead of a subdomain on mywordpro.com

The only way I found to map domains is to go in to the network site setting after registration and change the domain. Is there a way to do this automagically during the registration process?

My DNS and Apache servers have already been set up for wild card on a specific IP, so I know that part works.

Any help you could provide with be greatly appreciated.