That, currently, would need to be a customization for your site if you need to extend portal attributes. You could do it either by developing a custom module to do so, or you could modify the core code to do so. The recommended means would be to develop a module that does this for you.

Going through the settings for my dummy DNN site I can find no way to hide the portal (perhaps the expiry date under host settings could do this, but that would not be very intuitive) - is it possible to make functionality for this without altering the core?

I am unsure if we really need it. Today the manager wants to be flexible and have the ability to hide the "festival newspaper" of coming festivals when no content has been created yet. Today old newspapers are are always parked at the end of a festival season, so this is logically achievable today.

However, by merging the festival newspaper with the information site as we are thinking about doing - hiding the festival newspaper would also hide the information site AND all earlier newspaper pages. Therefore we probably would not be able to use this, and you assumption was correct;)

This highlights a challenge in information organization. It should be possible to park away last years content and make it easily accessible under sound categories. I will have to look into this.