I was in the Pyrenees last Autumn and visited the Maupas refuge, that had the most basic facilities that I have ever experienced. It was a basic cubicle with concrete drainpipe that (I imagine) led to some kind of treatment pit. It didn't even have a door, mind you the chances of being observed whilst in there are zero. If you take a look at the link below you can see said cubicle to the right of the main refuge, you get one hell of a view whilst having a dump
https://www.summitpost.org/refuge-de-maupas/336386