For my master's thesis, I designed/built a prototype that would track video modifications and add them to a signed report like this. Maybe it's finally relevant: http://www.dissertations.wsu.edu/Thesis/Spring2015/E_Houle_0...

Maybe with a chain like with SSL certificates & authorities? Or maybe there could be a reference database with all the versions signed by a given authority so that you could verify it's an approved version? Why not go even further and show the same kind of alerts / block content if the source / signature can't be trusted?

Not without losing the signature. But if you take a clip from a larger video you can (and should) refer back to the source.