Account-Wide Variables/Secrets

Is it possible for an administrator to define account-wide variables for users that later cannot be updated by the user? This variable would be used as a primary key for a SQL query in order to only show the user specific data. By doing so I would not have to create a separate dashboard for each user with the hardcoded primary key. As far as I know dashboard variables can be set through the URL and would make it possible to see sensitive data of other users. For this reason a different approach would be needed.

A similar question to this one has already been asked here, however the post has as of yet no replies.

Does anyone know if user-wide variables can be created or if there is an alternative approach to creating a dynamic dashboard that restricts access to other users sensitive data? Any help would be greatly appreciated!