I know you can do it by just removing the polarizer and putting it on your glasses like this: https://m.youtube.com/watch?v=xQ_pkWzy0_o

But it should definitely be possible to just replace your screen with a non standard filter that requires the right glasses filter to see so its even more obscure. 

Now the real prize would be if theres a way to make it so sensitive or multi layered so that it's not feasible to brute force the encoding. I don't see much of a market for this tho. Anyone this worried about physical privacy should be worried about digital instead.