Why does applying a new lantern impossible for CS?

[Romulator]

Couldn't you just put your lantern directory higher in the resources.cfg, and change some of the cfg's to point to that lantern?

Either way, the main reason for not using it in CS' is because that it interferes with the user's lantern. It would also mean that the custom lantern would be used in all CS' and the original Amnesia. You would want to include the original lantern for the sake of backing up if you release one with an edited model.