rendering another view isn't that bad. probably every refraction shader you've seen uses another view.

here's a quick idea, though: first view renders the scene normally. second view renders the scene with no textures applied, just grey for everything (no texture look-ups, single-line shaders... it's looking quick already!!), and you could even use lower LoD levels or a shorter draw distance if you are really pinickity. but it doesn't render everything grey. there are sprites that are basically "refraction maps" that say how much to offset each pixel.

the third view (don't panic! it doesn't even render the scene again, it's just a post-processing stage) takes in both those images (the rendered scene, and refraction-offset scene) and offsets pixels from the nice render based on the colours it gets from the second view.

but i can't do any examples because i have way too much work to do atm.

and i only just realised you still have A6, so forget about easy post-processing. i highly recommend you get A7.

julz