Anton - Thanks for reading and commenting. I've gone as far as creating a skeleton AutoValue extension to better understand how that API works, but I don't yet have a working prototype for either of these proposed additions.
I'll move on to prototyping the Coder generation for AutoValue classes if I get some clear signal from this list that maintaining an AutoValue extension for generating this code seems like a reasonable path forward.
I think this is a great idea! Thank you for working on the proposal. I left couple of comments in the doc.
Have you tried prototyping this?