From what I gathered, it requires a special set of circumstances that need to be met, as well as playing on a specific version of the unofficial patch plus, initiating the ending sequence with low humanity and an amount of other unverifiable various actions as well as going with any ending that lets you slash LaCroix's throat, and doing so; upon which the special dialogue option appears.
I ask this here, if anyone is able to provide with me a list of the most basic requirements to achieve this ending. Thanks in advance.
All you have to do is to go to Luckee Star Model, do what's written here, and by "open doors here often", it means really often, just spam with the action button. When it's all done right, when you go outside, you can see a giant werewolf dancing in the moonlight.
When it's done, during almost all endings -- excluding the one where you side with LaCroix -- when you're talking with him, you should see a dialog option: "Shut up and show me that dancing werewolf!" -- or something like that.
And that's it! Of course it works only with a patch plus.