That is a strange corner case and not something I will be implementing support for any time soon.
Generally if a site allows registration it is because it wants more users to register and the challenge is how to get more people to register. If we block a user because we only want to handle so many per day then we may not get another chance. I would never go back to a site that said sorry that is all the registrations we can handle for today.
Registering on a site is not the same concept as registering for an event and site registration is not designed like event registration.