We can disable automatic creation of new sites in a farm by setting server config parameters. Now we explore possible automated workflows that will replace this convenience.
The server can be configured to allow access to only specific sites. The wildcard symbol * offers access to all existing sites.
allowed: "*"
Accessing a non-existant site produces an "oops" message and an offer to view the parent site.
The wiki you are trying to access does not exist. You may visit crazy.fed.wiki for more information.Oops!
In support of this redirection we now offer a highly configurable workflow based on custom home page and a new server-side custom endpoint in the Register plugin.
We now have a workable Register plugin supporting the simplest "friends" workflows. We now consider how we might handle more nuanced situations without stumbling as we have in past proposals.
We have developed a demonstration site which uses these mechanisms to collect potential registrant emails with no real intention to provision sites.
I'm managing the crazy signups as an example of how this might be done with custom html pages. github 
This is a work-in-progress. Any potential user should contact me on any of our regular calls.
Federated wiki developers and users meet twice a week by video chat and all day every day by text chat.