Can't your rotate the portal's VIEW with the player's camera? Perhaps deforming/changing its shape so it doesn't look like you rotate the VIEW with the player? (just a wild quess)
Sure you can, and I guess that's the key to sucsess
But the problem is, that simply moving/rotating it won't help. I need some kind of algorithm which will calculate all those rotations/movement stuff automatically. F.e. Uhrwerk said this about rotation:
target portal view angle = camera.angle + (see through portal angle - target portal angle)
And I got it to work somehow, but without movement only rotation looks really weird (or maybe it just doesn't work correctly
).
ps: doesn't unreal 1 (not UT 1, but unreal) has also 'real' portals? Not does it matter much, but perhaps it can give you ideas.
Take a look at this. As the video describtion says:
Duke Nukem 3D supported non-euclidean geometry, in 1996.
Maybe Unreal had same non-euclidean geometry support, I don't know unfortunately. Tomorrow I'll make a small research probably, it's too late right now here at my place.
Edit: Here is what I've made so far right now (code is dirty and too many comments I know, just ignore them
):
Download link 
