Is it possible (as in, easy), to use something like gerrit/rietveld on top of (or next to) github?
I am imagining a bot what will take GH pull requests, submit the patch to gerrit for review, and answer to the user something like "thank you for your contribution, this project uses an external platform for code review, a discussion was automatically created to review your changes, please follow this link to participate".
Another option is to use a code review tool similar to Gerrit / Rietveld but built from the ground up for integration with GitHub. Recent entrants include https://review.ninja, https://gitcolony.com, and https://reviewable.io (disclosure: this last one's mine).
I am imagining a bot what will take GH pull requests, submit the patch to gerrit for review, and answer to the user something like "thank you for your contribution, this project uses an external platform for code review, a discussion was automatically created to review your changes, please follow this link to participate".