Problem is, this way journalists won't have anonymous sources anymore and you will be able to track any citizen filming power abuses.

Well, but then you'd have to trust the light sensor manufacturer to provide secure keys. Remember when the NSA hacked into Gemalto and had a chance to steal its SIM card keys?

However, that's still interesting; isn't it possible that each light sensor has certain watermark-like characteristics arising from the manufacturing process?

Yes, fingerprinting of digital cameras has been a thing for at least a decade [https://www.eurekalert.org/pub_releases/2006-04/bu-bur041806...]