I don't like the idea of a bot doing this. Nor do I think writing release notes at commit time works well either (too many stupid conflicts).
What's the issue with a bot appending a few lines to a release-notes section if it sees it the commit message? But yes, release-notes conflicts are a major pain and kills the streamlined patching workflow - submit->review->merge, frequently requiring extra "->fix->review" steps right before merge.
Most other groups using Gerrit that I know of tend to write their release notes right before releases. Sometime shortly before a release, they'll go through a full change of putting the release notes together. Then if any other things are done before the release, you just submit another commit to it.
Might not work well with so many committers and no strict central project management. It will become a big burden to go through all commits and pull out all relevant release-notes (actually doing what the bot will be doing, only in a non-automated fashion because the commit messages won't be as structured). But if someone is willing to do that... sure, why not :)