People say that this can be done, but wont be, because its buggy and would lag a lot.
people said that for Shadows and Shaders.
At what cost?
Depending on how the shadows & shaders update goes, it may be possible.
Kompressor posted on the S&S Topic that mirrors are NOT coming in game as the sheer amount of lag would be unbearable from the average player, Imagine having a 100k build on a computer that's lagging from it - Then putting one of these mirror bricks down, It'd have to render the whole game again so it reflects it or something like that
We used to have a mirror addon, but it was basically broken.
Wait, what?