At the time, someone said that the rain is just a "kind of screen" placed in front of the camera", not an actual mesh in the game.
One of the modders was trying to detect if the player was under some kind of roof, not so easy as many of the possible 'roofs' do not even have collision data.
What I did for that proof of concept video was to put a trigger box around the shack, so the script would know whether the player was in or out.
If the player was OUT, the rain would be normal. If IN, the script changed the weather to 'no rain'.
I also placed some vanilla meshes that looked like waterfall at the edges of the roof.
Some other things that may or may not help:
It only rains in exterior worldspaces (I don't remember if this is the term used), not in interior worldspaces. When you cross a door, you are teleported to an interior worldspace where the interior of the house is built.
Some modders where working on creating houses in the exterior, so you could look out the window and see NPCs passing by.
One of their problems was the rain. If any of them finished and published such a mod, you may check how the 'rain problem' was solved.