Unluckily, over than the SD software connection, HUD systems does have another (much more important) problem: to be useful, the data should be visible with focus set to infinite
. Just reflecting the data on a screen force the sight to focus at a shorter distance, loosing focus outside. Military (and the few civil) HUD does have a quite bulky optical systems to obtain this. There is no way to replace them with some kind of electronics, so an acceptable sized (and cost-effective) device for the vast majority of the SD users is now very hard to have. See picture below about an effective HUD system.