Custom Attachment Points can sorta do what you're looking for ... wherever that is on the website.
(Weaponlib has CAP built in as a function, but insert jokes here about "weaponlib did it" and "not compatible with [overhaul]" so CAP might be better for what you're looking for)

The optic attachment point is normally a_o on pretty much all weapons. Using CAP functionality, you would make a new CAP attachment point named a_o for [insert weapon here using its internal id], then modify the y value of the Vector3 position to move the optic however far up or down the rail you want

The last time I recall messing with CAP, I vaguely remember negative values in y would bring the optic closer to the camera/the butt of the gun in terms of centimeters (?), while positive values in y would move the optic away from the butt of the gun/to the muzzle of the gun

Example here with the AK17, would go in weapontweakdata. y would need to be replaced with a numerical value such as 20 or -7

self:SetupAttachmentPoint( "flint", {
    name = "a_o",
    base_a_obj = "a_o",
    position = Vector3( 0, y, 0 ),
    rotation = RotationCAP( 0, 0, 0 )
})