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is it possible to apply the negative shader only to a sprite or something, so only everything behind that "sprite" is negative?
Yes, this is possible (not really on a sprite I think, don't think shaders will work on sprites, but you could make a one-sided model). Infact, it's quite easy to do so, there already is Oliver2s his refraction shader example out there (somewhere) using that method to distort things behind windows.
Anyways the only thing you basically need to change, is change the view entity into a real entity, give it the material and then you should be done already. I think everything else can pretty much be left as is, camera settings included. 
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Now only if I could jump in time to two weeks when I purchase a6 pro for my uni course
I totally know what you mean, one of the main reasons I bought pro was the possibility to make post-processing shaders and realtime mirrors.
When you look at the code I've posted here, mostly it's color calculations, so it's really nothing complicated.
Things become really awesome when you combine stuff, like for example Ichiro's pucker shader thing and my nightvision 2 shader, shouldn't be to hard to figure that out.
Thanks for the comments guys, I hope this is useful,
Cheers
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