When I saw this, the first thing I immediately thought was: why don't webcomic authors use this to fix the problem of people linking directly to their images instead of the pages their images are on? This could revolutionize how webcomics and social aggregators interact.
Hotlinking isn't actually a big problem these days, or at least I don't hear anyone complaining about it. A bigger issue is people who take the comic, remove any watermarks, and then upload it somewhere else.
I don't know why they remove the watermarks, but that step alone invalidates your suggestion.
