You cannot adjust your stance using the stance modifier when you are using binoculars, any launcher weapon or no weapon at all.
It appears that the only states that support stance modifer are when you are holding a pistol or rifle. You can see this when looking at the Adjust animations in the animation viewer.
Press B to bring up binoculars
Press Ctrl-S or stance modifier key to lower stance
Result: No action
Yeah, trouble is, all those alternate stances are done by brute force - its just tons of extra animations. As binos and launchers are not so widely used as rifles and pistols, alternate stances for them won't be implemented for Arma# release. mazbe in the future when we wouldnt have anything else to do, but I doubt it because we will be thinking of other cool stuff