Yes, I did allow for that interpretation which was my third paragraph from the bottom.
So to rephrase, there are many individuals who do such things. But there seems to be two sorts of these peoples.
Retrants in a retreat house or monestary and solitary retreatants.
Now retreatants in a monestary or retreat house do this for perhaps very long periods of time, or I'd guess it could be a permenant thing, but that would be rare. The most common retreat, is three years three months and three days but many do longer periods. So you may have someone in such a place in virtual permenant retreat but that is differing from a solitary retreatant who basically has cut all human ties. Solitary retreatants generally will stay in silence and have no inteaction with others except to pick up foods left them by others. Never even seeing another person for the most part. In a retreat house or monestary generally the retreatant stays in silence but still there is nonverbal interaction and customarily pretty close interaction with the teacher or lama.
So there are two different kinds of this similiar peoples.
Is there a retreat house in France affiliated with Kagyu....I'd assume so. A quary to start would be perhaps Khenchen Thrangu Rinoche's web site, and if not affiliated with him, links or information there, may be a place to start. A general quary on such a thing may give a answer as well.
Are there solitary retreatants in France...I'd assume so as well. But they do not want to be known of and are often know of, only by their teachers or benefactors. They most often stay in silence, and limit any human interaction, so to have their locations known of, would be counter productive. But again this is not normally a group practice setting, though they may be close to one another physically. In Tibet and other places there may be caves close by to one another for solitary retreatants but this does not mean they are practicing in a group. So that could be a situation elsewhere. A benafactor may have a series of huts relatively close together but with the habitation, solitary retreatants. But they are not practicing as a group nor do they communicate with one another. Preferablly I'd say they would be totally isolated from everyone, but functionally that may be hard to arrive at and still eat and drink what is necessary, especially in a western environment. Even interaction with a physical teacher may be nonexistant depending on how long they have been in solitary retreat. The preference is to be solitary in every sense of the word.
Walled off solitary retreatants is another form of this....I don't think any in the west or perhaps anywhere do that thing currently. Most commonly in monestary in historical Tibet, a retreatant would be walled into a cell for meditational purposes on occasion for very long periods of time. But I would guess that is not much done nowadays.
I am just a layperson with little understanding or knowledge of such things but in my limited fashion I have tried to answer. Certainly more educated if they find this thread could provide better answers.
"This order considers that progress can be achieved more rapidly during a single month of self-transformation through terrifying conditions in rough terrain and in "the abode of harmful forces" than through meditating for a period of three years in towns and monasteries"....Takpo Tashi Namgyal.