This is what I've come up with so far. I'm not a very experienced bash programmer, so I'm sure there are improvements that could be made both for readability and efficiency, but this is what I got to work after hours of finagling.
xset s off # This turns off the "screensaver" feature (not actually a screensaver, just an annoying power saving feature.)
xset -dpms # Turns off dpms (desktop power management system -- also annoying)
while true; do
gsettings set org.gnome.desktop.screensaver picture-uri /WALLPAPERS/"$(ls /WALLPAPERS/ | sort -R | tail -n 1)"
sleep 30 # This is the amount of time in seconds to wait before changing
Where "WALLPAPERS" is the full path to the parent directory of whatever image(s) you want to set on the lock screen.
Save this script with whatever name you want e.g. lockscreen-slideshow.sh -- just make sure you know where it's being saved to. You'll also need to make it executable, which you can do in a terminal with
chmod +x lockscreen-slideshow.sh
Lastly, you'll want to add a startup script so you don't have to open a terminal and run your script manually every time you log in, so make a new file in
~/.config/autostart/ e.g. "lockscreen-slideshow.desktop" and drop this copy pasta into that bowl, then modify it to reflect the right path.
Comment=fetch a random anime wallpaper and plaster it to the lock screen
Something to note: this is a slideshow in a very loose sense. It'll just swap the images out instantaneously. I haven't figured out how to make them transition smoothly yet, but I'm playing with adding new keys to gsettings schema (and then subsequently copying the key from org.gnome.nautilus.desktop called background-fade over to org.gnome.desktop.screensaver).
Hope this helps for now.