Is there a namespace for the collection that will only be used within the organization?
We do not plan to upload any collections to Ansible Galaxy or Automation Hub.
However, for the purpose of using documentation tools (such as anstibull-docs), we want to treat the roles we have developed as collections. To do this, we need a namespace.
Namespaces such as test or private seem to not yet exist; local exists, and is somehow associated to @geerlingguy and @gundalow (Ansible Galaxy). I hope local has been reserved for exactly the purpose that you ask for, but I cannot find it documented anywhere.
I have confirmed that an addition has been made to the section Naming your collection in page Creating collections in the official documentation about local namespace.