Menachem,
Very bad comparison. The Chafetz Chaim, zt”l was the undisputed leader of world Jewry and his emphasis on Lashon Hara was first and foremost about these vital halachos which were being ignored.
The Rebbe, zt”l was definitely not the undisputed leader of world Jewry, contrary to Chabad PR and in most areas was considered a daas yachid. Regardless of him mentioning it once or hundreds of times, Hakhel may be a nice Chabad custom and a big “Kiddush Lubavitch” (whatever that means) but it has zero halachic relevance to the rest of us.
With love, please proselytize elsewhere.