I don't think that's what Emily was getting at - and it's a good point, that I hadn't thought of before. The outside of the helmet needs to be able to slide and not catch on anything. Same principle as why the shell-less helmets and the extra-long aero helmets were taken off the market.

Maybe one of those cat collars that are designed to break off as soon as they're caught on something, could be modified to mount a light on a helmet?