You can create a folder structure, and then set the access controls (ACL) on the folder structures so that users cannot change the first few levels of the folders, but still able to make changes to sub-folders.

You can also give privileges to some users to manage folder structures, which will give them the power to override access controls. In other words, these people will become the administrators of the file plan.

Perhaps you can provide more details and examples of your meaning of "file plan" and "centrally defined policy" so that someone can give a more specific answer?