Just an idea. Not sure how good it will work.
Setup a camera that points to the destination area. Positioned at the portal exit. Manipulate its properties (FOV, rotation…), based on the original player camaera - entry portal values. Ie if your original camera is at 45 degree angle with the entry portal plane, you adjust your secondary camera to be at 45 degrees with the exit portal plane. Then render the view given by the secondary camera onto a render target and use it to texture the entry portal surface.
I had tried the above method in UDK and got pretty good results. However it was simply a demo I did after playing portal and did not have anything to do with gameplay. SO I cannot say anything about how it will affect performance.