Hi Rick,
Thanks for taking the time to suggest this idea.

Something like this should already be possible by customising the default page on your webserver. For example in cPanel: https://forums.cpanel.net/threads/there-is-no-website-configured-at-this-address.47309/

Then in WHMCS you just need to set the default nameservers to point to that server (Configuration > System Settings > General Settings > Domains tab).
New domain registration will then see that default page without any hosting.