It looks like the main fruit of this is piet-gpu[1]. How close would you say it is to stable? And how complete a realisation of the concepts put forth in the OP? I'm currently working on a ui library, and evaluating alternatives for hw acceleration. The whole library wants to be in c, so I would want to rewrite the library by release-time to use it, but don't want to waste time doing that now if it's still really volatile.
Not very, and not very. This is research, it depends on compute capabilities that have really only gone mainstream in GPU lately, and the current codebase is about trying out the ideas. Best of luck in your project!
1. https://github.com/linebender/piet-gpu