The fullblown case is difficult, true. But the 'simple' case would be that one of the forks has a repo. Afaik Zen already has its own mods: https://zen-browser.app/mods/ (I did not check the details). Coupled with GitHub attestation packaging more FOSS from trusted sources, would maybe not easy but also not implausible difficult. The difficulty for sure is to set up some trusted moderation community. And I think with mozdev.org, the future was partially already here, before all the browser wars...