Yep, the portal information is totally stored in the file. Now which classes do I have to inherit/edit to do portal rendering? Or do you think you can put it into irrlicht in... say max. 1 year?
I think a general "occlusion/renderbehavior" interface would handle some common problems which have to be solved. For example the occlusion by portals or BSP or LOD or Frustum etc. Maybe something which allows to set up different techniques (also costume written) with an order like "first BSP, then Frustum, then portals". But its more a guess and not a worked out idea.
Portals are very easy to make, you just have to use an RT camera pointing i n the view of the other portal. To work around it looking flat, rotate the camera with the player's camera.