You can get various "harnesses" for a GoPro or similar action camera:
https://www.amazon.co.uk/DURAGADGET-.../dp/B01EYNVQFQ
https://www.amazon.co.uk/GotiTronics.../dp/B01MT9JYFU
https://www.amazon.co.uk/USA-Harness...=action+camera
Obviously (a) it ain't too subtle (especially the head one) and (b) you need the camera and be able to easily turn it on and off.
There are things like this but whether they are more than a toy, I don't know.
http://www.ebay.co.uk/itm/Mini-Butto...r/222409339516