I tested this and it works perfectly well for me, an example configuration is shown e.g. at:
olifre/olifre.github.io@bff63b4
It does not seem to work with the standard Jekyll builder on GitHub, but that's probably since it seems to ignore selecting a specific branch. Works fine like a charm when testing it locally, including generation of appropriate robots.txt and allowing to change name of the index file.

Thanks! 👍

I tested this and it works perfectly well for me,

Thanks for testing, @olifre. Happy to know that the current implementation works for your use-case.
However, I wouldn't recommend using in production until this gets reviewed / merged at minimum.

However, I wouldn't recommend using in production until this gets reviewed / merged at minimum.

Sure 😄. I was just trying it out locally and checked whether it would work with GitHub Pages, the commit right after the one I linked reverts the changes and I am using a manual workaround (hand-crafted files) for the time being.
Thanks again for the implementation!

Just pinging this – this is really useful, thanks again for submitting it. Could this be merged? (Who should we ping for a review?)

bumping this