This is probably obvious, but if you modify the theme to support landscape lockscreen orientation, the wallpapers will be shown on the left half, cropped for a portrait screen. Even setting an image as wallpaper will ask you to crop it. Set a custom wallpaper, then overwrite the image in /home/user/.wallpapers/wallpaper.png with an 854x854 image and you can get full wallpaper in either orientation.