Why do you need wireless? Seems pretty straightforward: put a photosensor of whatever type on the optic pointed downrange. Alternatively, put the sensor on the user's side of the optic, embedded in or near the BUIS also pointed downrange, so you can adjust what the user is actually seeing.
From another standpoint, you are adding even more electronics (and possibly software) onto a package that is trying to optimize size, power consumption, and robustness already.