Yeah there's nothing that prevents you from using a tool in exactly the opposite way that it's designed to be used, but it's also pretty unlikely that it's going to happen or that it's going to be successful.
Just make a PR early and discuss the code as you build and make changes, not sure in what way the tool wasn't made to do that. Then you also get the discussion interleaved with changes, resulting in basically perfect documentation of how the code was made and why.
The only thing PR's lack in that regard is that if the reviewer accepts it then it gets added automatically, while ideally you should re review things after the reviewer has accepted it. That way they wont accidentally accept prototype changes.
I'm beginning to think you've never actually used github. Are you trolling?
The tool does not stop you from working in the way you suggest. Maybe it's not what the engineers who wrote it originally envisioned, but it's both simple and easy to work in the way you suggest.
If I want early feedback on something I create a draft PR and ask for comments. I can then let people consider my approach asynchronously, in their own time.
