If all someone has to do to donate to an artist, is scan an extra QR code at checkout or ask to have their cash converted to cryptocurrency
So, IIUC you're suggesting the QR to be a (hopefully reliable) way to find the author so as to be able to give him money?
I can see why that would be nice, but I don't understand what the cryptography is about (i.e. how will it increase the trustworthiness of the QR)?
How would you detect that someone took that video, removed the author's (cryptographic) signature, and put his own (cryptographic) signature on it? Signatures are not useful for that. You'd need something more like "watermarks", but here again the benefit is likely much smaller than the harm.
Better just slap a URL/QR inside the video as part of the opening/closing credits.
Also instead of watermarks you could try and develop a kind of registration/certification body, so when the end-user wants to donate to A for the movie M, the donation site could warn the user "hmm... according to <foo>, movie M was not directed by A but by B, do you still want to donate to A?"
Stefan