After talking to Dany, we figured out a way to do this: The file could be marked in the file index table on the server as downloadable/visible only by a single user-- When everyone else does a file index, they don't see this file. When the user in question does a file index, they see this file.

This could be used for user-specific settings that an application wants to save-- e.g., the read state of specific discussion threads. This would enable that read state to be preserved, even if the app was deleted and reinstalled. And would enable that read state to be used on a different device, by the same user.