hmmm...well maybe something in the line of that a script registers your current crosshair target (like the BIS camera script feature to make it easier to find camera positions), and changes your current target point + an amount of the way you breathe.
So basicly itll need a script that changes "current target position" +# in height in an interval of 1 seconds, wait for 2 seconds and then changes -# in height. And this continously happening when aiming, realtime.
Im a beginning at scripting....so I wont be able to indulge into this, only give a few ideas