I'm working on a protocol for 2D portals in Unity using render textures and and dynamically generated meshes. I've got good results so far with the rendering; you can play it (and check up on progress) here.
It's called Unity Portal Protocol 2D, or UPoPr 2D (because it 'pops u around', get it?), and I might release it and/or put it in a game when I'm done -- depends on how easy it is to work with.
I'm releasing updates every few days :)