Hakhel: Source for leader of generation replacing king

Question

As this is (or would be) a hakhel year I was doing some reading up on Hakhel and came across the following statement in a chabad.org article:

If there is no king – as was the case in the hundreds of years until Saul was anointed –the leader of the generation reads.

I can't find a source for that anywhere, the chabad article doesn't cite it. Any idea where it's from? I've checked the obvious places - Rambam, Sefer HaChinuch, and a few relevant sugyas in the Talmud but I can't find it. Any idea where this halacha is from?

Just to clarify I realize there may be historical precedent from the story of Ezra, but I'm looking to see if/when this was actually codified into halacha.

Answer 1

Minchas Chinuch suggests it:

והנה מבואר בש"ס דהמלך הי' קורא וא"י אם הל"מ כ"ה דדוקא מלך ואם אין מלך בישראל אין מצוה כלל א"כ עד שאול המלך עליו השלום לא נתקיימה מצוה זו ואפשר דל"ד מלך רק גדול שבדורו עליו היה המצוה אם לא הי' מלך וכ"נ מצד הסברא

Now it is explained in the Gemara that the king reads, but I don't know if the Halacha Lemoshe Misinai is that it's specifically the king, and if there's no king in Israel, there is no mitzvah at all. In that case, until Shaul Hamelech Alav Hashalom this Mitzvah was not fulfilled. It could be that it doesn't have to be a king, just that the greatest person in his generation was obligated to do the mitzvah if there was no king, and this seems logical.