[Tymon Douglas]

"Quite simple"
Too bad it isn't as simple as you think, because for the game to load a skin it must be in the same format as all other skins, so at some point they'll have your files anyway.

-->"as long as it is possible" <--

Alright,
lets add a rule where you can report someone for skin piracy, because it is also possible to take screens ingame and pretty much copy the skin, so yeah, seems useless such a public/private feature indeed...

The only effective way to stop this is actually doing that, reporting for using your personal skin. The way assetto corsa works doesn't allow any other solution. It needs it to be fully readable on the skin folder for it to be shown.
On another note, i'd prefer to be able to upload my skin beforehand instead of just being able to do that during the race, because my bandwidth is quite limited and sometimes i'm not able to upload it on time for it to show up on stream.