I see what you mean; but to be honest I am not sure if it’s worth it (in the sense that every feature increases the complexity, and the time needed on both sides, devs and users, to handle everything correctly). But there is this limitation, currently, as you correctly observed. It may be smaller than you think though, I’m not sure:
What you can do currently is
- Create an automation user to bake agent packages that will automatically create the host object in the monitoring and register themselves when they are installed on a host. The secret contained in the agent package can not be used to re-register. Ultimately the site decides which hosts to create where (hosts can not be overwritten!).
- Create a user that can only register to existing hosts, but not create hosts in this automated manner.
What you can’t do is create a user that can register to an existing host, but only once (or only the first time, which is not the same). You could create a user that is allowed to register, and then just remove it after a short period of time…