Ok this isn't a mod as such, it's a file rename system.
Make a copy of the armor folder as a backup (so if you mess up you can just delete the armor folder you are working on and just rename the backup one armor again to put the game back to normal)
Go into the armor folder and make 3 copies of the underwear folder (this is naked / base loincloth based on nudity settings).
After that rename the folders named below
light_exile_cloth to light_exile_cloth_backup
medium_studded_leather to medium_studded_leather_backup
heavy_plated_leather to heavy_plated_leather_backup
Now rename the 3 copies of the underwear folder you made earlier to the 3 initial folders named above and all these armors will show as nude / base loincloth when you go into the game. Including thralls and npcs.
There are some bugs still with blacksmiths etc but just needs more folder changes no doubt, if you find out how to fix that please post.