This is possible but I was under the impression that it was not. Possibly I have been reading old posts or not reading the right posts.
I've recently been trying out a Youview DTR-T2100 (Humax) and a Youview DN372T (Huawei). Both can unhide individual channels providing the STB is connected to the internet.
To unhide a single channel:
* Identify a name of a programme that is coming up on the channel that you want to unhide;
* If it isn't already connected connect the Youview box to the internet;
* Search for the programme using the youview search;
* On the search result list switch away from On Demand and adverts to the 'On Now & Next' list of programmes;
* Select a programme from the hidden channel to record using the OK button;
* Enter your youview PIN (default 1234);
* A message will pop-up briefly stating that Youview is currently unhiding the selected "channels", and your selected single channel will be restored.