I'm not sure if this already exists

There should be rendering layers or groups, that you can flag your game objects in GED/WED. Then you could control larger part of your scene. You could hide a section, without accessing each one, until an action enables them, you could assign post process shader, such as bloom to one section, and SSAO to other. Also you could determine which are indoor and which are outdoor stages (helping out weather effects)... The list goes on.

If such feature already exists, I'm eager to learn it, as I really would like not to constantly use invisible portals when creating environments and limiting passages, etc.

I guess this could be done via a general render event and various cameras.

For preventing the rendering of entity groups in a certain view, the simple method is using NOFLAG1, the more complex method is assigning a material with a render event, and the Pro method is using portal regions. I think these 3 methods cover all possible situations.

Thanks. I'll look into the material and NOFLAG1.