I never used to wear a helmet on my bike either, but have taken to wearing one as I started doing longer distances. In truth, the safety aspects of cycle helmets are over-stated as they don't really provide that much protection against typical bike accidents. I always think that gloves are far more important on a bike than a helmet as pretty much any time you come off a bike, you'll use your hands.
Anyhow, as I wear clear protective glasses when cycling, I wouldn't have any problem with a similar design for a HUD. I imagine that the higher speeds of a motorbike might make a difference, but I've seen bike riders wear prescription glasses under their helmets, so I don't think it's an insurmountable problem.