I tried to code an example to this but couldn’t. Hugo simply doesn’t recognise weeknumbers (as you already pointed out), which of course makes sense because based on the source I looked at, Go doesn’t either.
There’s still a workaround, however. If you add the weeknumber to the front matter of each post, you can use that value as a key to group by. Of course, that’s not great since it’s a hassle to figure out the weeknumber and to ensure you always provide it in a post.
Thanks for trying. I thought about add weeknumber to front matter, but it’s not convenience, neither organize manually, I don’t have to deal with lots of files each week for now, so I decided to organize by hand now.