I think it will be better to use another tool than Rietveld because since the decision of Python to stop using it and migrate to GitHub, the development has staled.
There are not so much alternative software that works with Mercurial and as pre-commit (see: https://en.wikipedia.org/wiki/List_of_tools_for_code_review).
RhodeCode: Provide full hosting and work with PR. This would be a big workflow change.
Phabricator: Also do more than codereview, written in PHP which will require new knowledge for hosting.
Patchwork: Based on the interesting idea of mailing list. But without the "send reply to ML" it will be not very easy to use for new comers.
Review Board: Describe itself as alternative to Rietveld. It also has bot: https://github.com/reviewboard/ReviewBot
I must say that my preference is going to Review Board.
I propose to setup an instance on our host to test it.