I had forgotten about the film-grain extraction, which is a clever approach to a huge problem for compression.
But... did I miss it, or was there no mention of any tool to specify grain parameters up front? If you're shooting "clean" digital footage and you decide in post that you want to add grain, how do you convey the grain parameters to the encoder?
It would degrade your work and defeat some of the purpose of this clever scheme if you had to add fake grain to your original footage, feed the grainy footage to the encoder to have it analyzed for its characteristics and stripped out (inevitably degrading real image details at least a bit), and then have the grain re-added on delivery.
So you need a way to specify grain characteristics to the encoder directly, so clean footage can be delivered without degradation and grain applied to it upon rendering at the client.
You just add it to your original footage, and accept whatever quality degradation that grain inherently provides.
Any movie or TV show is ultimately going to be streamed in lots of different formats. And when grain is added, it's often on a per-shot basis, not uniformly. E.g. flashback scenes will have more grain. Or darker scenes will have more grain added to emulate film.
Trying to tie it to the particular codec would be a crazy headache. For a solo project it could be doable but I can't ever imagine a streamer building a source material pipeline that would handle that.