The only thing that can be loaded from command-line is a processing profile. You can create a processing profile which uses nothing but an LCP.
- Open a photo.
- Apply the Neutral processing profile.
- Apply your LCP file.
- Save the processing profile, e.g. “
Nikon_D810_Nikkor_24mmf14G_ED.pp3” (good idea to put the camera and lens names in the filename). To make the processing profile contain only the LCP entry and nothing else, partial-save it by ctrl+clicking on the processing profile save button and select only “Lens Related Settings > Lens correction profile” - Use that processing profile from command-line.
http://rawpedia.rawtherapee.com/Sidecar_Files_-_Processing_Profiles
http://rawpedia.rawtherapee.com/Creating_processing_profiles_for_general_use
http://rawpedia.rawtherapee.com/Command-Line_Options