tmpfs is in memory it is not taking any of your emmc space. So simple answer is you can not use it to increase your root space.
So if your thumbnails are really eating up the 11G that you besides the OS normally have free on root that your only option is to integrate a SD Card or USB as the storage for your thumbnails.
Thanks @fzinken this my option. I have already wrote the Kodi skin forum to fix the issue as well… without having local artworks I am getting Blank screen.