I've been trying to get the lockable slots to save for a while.
Another modder, much smater than me (FuriousRamsay) managed to get it to work.
So, lockable slots now save, and are set back to what you had on game reboot. This screenshot? This was after setting to 8, closing the game completely, and rebooting it :)
So huge thanks to FuriousRamsay for saving me hours of smashing my head on a wall.