APPLYDREAM command line parameter will make DS apply something BUT it reads what to apply from the registry (DreamToApply value)
That does not really accomplish what you likely want.
There are some other options we are considering for a future version of DS.
Stardock Support Manager
Well, it's not optimal to have to edit the registry every time, but I can easily script this. Thank you!
For future reference, calling DeskscapesConfig.exe APPLYDREAM will read the DreamToApply key in Computer\HKEY_CURRENT_USER\SOFTWARE\Stardock\Deskscapes, set the wallpaper to the specified file path and clear its contents afterwards.