I’m not sure about the pull request workflow though, how would that work for review? Is your concept more for collaboration?
https://joss.theoj.org/
* It's owned by a (single) commercial corporation, Microsoft.
* There is censorship both by content and in some respects by country of origin.
* The code is closed.
but otherwise it's an interesting idea.
The C++ standardization committee uses GitHub to track papers submitted to them, see:
https://github.com/cplusplus/papers