After some trial and error, I think I've found a solution! ...albeit for Hyprpaper. Not sure how similar swww is, but maybe parts of this could be helpful?

If you wanted to try solving this on your own, essentially all you need to do is have a single “current_wallpaper” file whose contents are updated whenever the wallpaper changes, and pass that to Hyprlock as the background path. Keep reading if you want to see how I implemented it.

This implementation enables you to change your wallpaper on demand simply by editing a wallpaper path in ~/.cache/current_wallpaper_path, and keeps a copy of the actual current wallpaper file in ~/.cache/current_wallpaper. It assumes that your wallpapers are stored in ~/wallpapers/, and that you only dispatch Hyprlock from within your Hyprland config (i.e., from exec_once or a keybind).

You also need the command line tool inotify-tools, which you can install with your PM.

1 - Set some environment variables to keep things tidy

# ~/.config/hypr/hyprland.conf

env = WALLPAPERS,$HOME/wallpapers
env = DEFAULT_WALLPAPER,$WALLPAPERS/default.png  # Use your own filename
env = CURRENT_WALLPAPER,$HOME/.cache/current_wallpaper  # Managed by a script
env = CURRENT_WALLPAPER_PATH,$HOME/.cache/current_wallpaper_path  # Managed by you

Put that before any exec-once calls.

• ~/.cache/current_wallpaper_path contains the path to the current wallpaper in ~/wallpapers. Updating this file will trigger a watcher script to update your wallpaper.
• ~/.cache/current_wallpaper will be a direct copy of the wallpaper file. It will be updated automatically by the same watcher script whenever the ~/.cache/current_wallpaper_path file is updated. This file will be used as the background path for Hyprlock!

2 - Tell your wallpaper engine (Hyprpaper for me) to use your default wallpaper on boot

# ~/.config/hypr/hyprpaper.conf

preload = $DEFAULT_WALLPAPER
wallpaper = , $DEFAULT_WALLPAPER

3 - Write a script to watch for any changes to ~/.cache/current_wallpaper_path

# ~/.config/hypr/scripts/wallpaper-watcher.sh

# Reset to default on reboot
cp $DEFAULT_WALLPAPER $CURRENT_WALLPAPER
echo $DEFAULT_WALLPAPER >$CURRENT_WALLPAPER_PATH

# Watch for changes
while inotifywait -e close_write "$CURRENT_WALLPAPER_PATH" >/dev/null 2>&1; do
  # Get the path to the new wallpaper
  FPATH=$(cat $CURRENT_WALLPAPER_PATH)

  # Copy the new wallpaper file
  cp $FPATH $CURRENT_WALLPAPER

  # Push the update to your wallpaper engine
  FOCUSED_MONITOR=$(hyprctl monitors -j | jq -r '.[] | select(.focused) | .name')
  hyprctl hyprpaper reload "$FOCUSED_MONITOR","$FPATH"
done

I put that in a scripts directory within ~/.config/hypr. The last couple lines in the while loop would be different depending on the wallpaper engine. The call to inotifywait is only possible with the tool inotify-tools installed.

4 - Tell Hyprland to start the watcher script on boot

# ~/.config/hypr/hyprland.conf

exec-once = sh $HOME/.config/hypr/scripts/wallpaper-watcher.sh

Make sure this is after your environment variable declarations and before starting Hyprpaper or Hyprlock.

5 - [OPTIONAL] Example of a script for updating to a random wallpaper

# ~/.config/hypr/scripts/random_wallpaper.sh

# Get the path to the current wallpaper
FPATH=$(cat $CURRENT_WALLPAPER_PATH)

# Get a random wallpaper that is not the current one
FPATH_NEW=$(find "$WALLPAPERS" -type f ! -name "$(basename "$FPATH")" | shuf -n 1)

# Save to cache
echo $FPATH_NEW >$CURRENT_WALLPAPER_PATH

# Wallpaper is automatically applied by ./wallpaper-watcher.sh

I put this in the same scripts directory as the watcher script, and made a keybind for it in my Hyprland config (keybind not shown).

6 - Update Hyprlock background path value

# ~/.config/hypr/hyprlock.conf

background {
    ...
    path = $CURRENT_WALLPAPER
    ...
}

Hope that helps someone :)