The collimating effect for the reflex sights is there already, the parallax effect with turning the head is also present in the game. What I ask is only enabling it in "aimed" view. It would affect only headtracking users anyway, as without such equipment the head is coupled with the weapon's muzzle and the ironsights are always aligned properly (unless [Alt] freelook is used).
I had an opportunity to check this with a real red dot sight. With my cheek firmly pressed against rifle's butt and without moving of the rifle itself, any significant head rotation caused the dot to "move" outside of the reflex glass. Small rotations only "moved" the red dot to a different portion of the glass, but it was still visible and allowed for aiming.
Edit: For clarity, have a look at this article about reflex sights. It describes how do they work and what are their advantages over traditional ironsights. http://en.wikipedia.org/wiki/Reflector_sight