I was thinking of a fully private deployed within the customers cloud account connected to the software provider via Private Service Connect or PrivateLink and then either only available inside the customers private network or only via their own firewall. The point being that it's not publicly exposed.
Since all your metadata and env vars live on the control plane, any issues there could affect the BYOC cluster.
There are some other options for a separate control plane, e.g. now they seem to have the term "forward-deployed control plane" which might be fairly close to what you're thinking of, but their BYOC by itself does not get what you want.
a hacker might know how to hack your service, but they'd have to find it first.