Hmm... that's very useful,ndeed. But it cannot cover just every case. For example. I have a stairs that lead to upper "level" of the building. I have a silent teleport there. On the upper section of that stairs is a door, so it need to be seen from lower "level" too. When the door is closed all is normal, since inower level it can be drawn by placing wall texture on solid wall, but when it is open, then teleporting from upper level to lower one you'll have a bizzare effect of door sudden closng (and vice-versa - opening). Of course, I may create a door on lower level as well, but then I'll have to make at least visible part of what lies beyond, but my map structure denies doing so, because there simply is not enouph space to do it.
And, anyway, I just thought it would be great also to make a sort of Portal-styled teleporter with ability to see place of teleport destination (that's how it was implemented in Quake 3 (if I remember it right).