Hello all (my first post, so I'll say that)...
I've implemented working and visually-correct Portals in BGE. As far as I can tell, they look like and work like Portals in Valve's awesome game of that name. Any object will transport (smoothly) through them automatically, preserving (but re-orienting) linear and angular momentum. The Portals themselves can be moved and reoriented as you please.
Basically the implementation uses VideoTexture.ImageMirror and a little maths. I'm no mathematician, so it took a few hours to get right, so I thought I'd save you all the trouble.
The script is in the attached blend file. You can use it in your own projects under this license:
Arrow-key around and enjoy "thinking in portals".