Nope, I really think this is a dodgy benefit that they try to avoid or not give.

Just connect to the premium internet then ask for the charge to be removed right after (before you checkout) for peace of mind. This is the only real foolproof way at most places.

The way the system set up at the moment doesn't really recognise that you're diamond. Even a diamond only access code connects you to the same internet - they lie a lot and say your room is registered on premium internet when you're using the same internet as all guests.