If you set up the hidden network correctly, it should show up in the list of networks while you scan. (I have all my networks set to hidden, ad I all find them this way.)

If they don't show up, maybe there's still something wrong with the way you set them up. I don't have my N800 so I can't help you with the details right now.

Oh, BTW:
You don't have access to the router/access point of any of these networks, do you? You could try to make them vidible only for a while, connect to them, save the connection, ... just to make sure that in principle you can connect and there's no other problem we just don't see yet because you can't make your tablet even try to connect.