I was also looking into changing the wallpaper depending on live or non-live mode.
To change it via the command line you can use the code posted here:
qdbus org.kde.plasmashell /PlasmaShell org.kde.PlasmaShell.evaluateScript 'var allDesktops = desktops();print (allDesktops);for (i=0;i<allDesktops.length;i++) {d = allDesktops[i];d.wallpaperPlugin = "org.kde.image";d.currentConfigGroup = Array("Wallpaper", "org.kde.image", "General");d.writeConfig("Image", "file:///media/sda2/Background/SpaceWall/Escape_Function.jpg")}'