This would be extremely nice to have as it would allow for animated alternative sights or changing the apperance of a sight when you are actually aiming down the sights.

New sources would be similar to current eventhandlers:
isADS - 0/1
opticsMode - index of current optics mode starting with 0.

These changes would be excellent for scopes/attachments too but I think that's a whole other can of worms.

Examples of usage:
Changing the apperance of an aperture sight when looking down it, while maintaing the correct physical side - ie. wider field of view, better representation of how the human eye sees through it. RHS does this already by having an optic attachment be added when aiming.
Alternative iron sights - Night sights, AA sights, a lot of weapons (particularly older ones) have different sights fit to the weapon. This can be handled with the 'zeroing' model source currently but not always fiting