Any object that doesn't use the 3D engine to render (ie. Plane ID's, the mouse, etc) will render at that "zero point" you speak of. This is because there is no z-value for the system to consider when rendering the left and right eye. Kind of cool though, feels like you are wearing a HUD!

This *probably* includes the gunsight as well.
As far as I can tell, the gunsight image is rendered in 2D, and always at the center of your screen (when looking forward). The only time it is visible is when the center of your screen crosses over the reflection glass. This would explain the issues people are having with the gunsights. Irregardless, if there isn't a distance value that the system can use to assign it a position on each eye, then it will show either "double" (because it is centered in both eyes, which would explain why it would be different aiming left or right eye) or at zero point.