I'm starting to think it's impossible with the Steam version, or something, but since I've found no keyword results with 'steam' other than Davo asking if somebody's install was a Steam copy, I figured I'd ask. So I've spent the last two hours googling, searching forums, etc trying to figure out how to do this. Pepper parts on Steam., but NO other mods installed.
TL DR - Force Save isn't working, and no matter how I do it I cannot save any of the custom parts from the Dark Injections Mod.