To measure the usefulness of the solution, ask your self the issue of the problem. How often do people really steal textures through custom folders, and those that do get banned. trying to prevent people from having access to custom folders would be protecting against racism by making it so you cant enter the words.
Not supported.
I remember once I dueled a guy for his set, and he didnt wanna send it to me, even though I owned it, so I just got it through custom folders.
If its watermarked that kind of scam might be more common.