What do you need really?
If you want to get a folder visible through USB but invisible through wi-fi, I don’t think it’s possible.
But if in fact you want to have a secure folder visible for you only, it’s possible using SSH. Just perform an SSH access and create your folder outside the Public folder. You will be able to work with it through SSH only (and entering a password therefore).
It’s not a good way (you’ll have to be carefull to avoid losing your files because of firmware updates and corrupting MPW’s system files) but is the only one for now. We are waiting for a regular tool from WD. Perhaps it will appear in future firmware versions. Add your voice for such a function in the Ideas section.