The directory you need for this is
The are not stored there though; this is the place to put them to make one read-only. From dconf admininstration ...
Lockdown is supported by creating a 'locks' subdirectory in the keyfile directory (ie: /etc/dconf/db/local.d/locks). The files contained in this directory contain a list of keys to lock, one per line. Just as with the keyfiles, you may add any number of files to this directory. Here's an example of what one might look like:
# prevent changes to the company wallpaper
Of course, you need to run the 'dconf update' tool after changing the locks. The tool will notice if the timestamp on the 'locks' directory has changed, so there is no need to touch the ".d" directory as well.
The lockdown key for hide-mode should be ...
I would consider it a bug if this does not work.
You could add this to some script or ~/bashrc or something else to get it reset.