It is currently not possible to review the commit message of a change request, short of adding notes in the free-form comment field. This is not enough, as commit message often have multiple lines and you then need to copy the full message in to add your notes, or otherwise find some way to reference what you are talking about.
Gerrit is much better in this regard.
Please provide a more complete background on the problem you are facing here. It's not clear whether you are trying to using Audit, or are having issues with Arcanist/Differential.
See e.g.: https://phabricator.kde.org/D3040 <-- I want to comment on the commit message, just like I can comment on the actual code changes. This is not possible.
In Gerrit, the commit message is part of the code changes, as it should be. See e.g.: https://codereview.qt-project.org/#/c/183631/1//COMMIT_MSG
Please note that Phabricator is not Gerrit - what you seem to be after is a very, very Gerrit centric workflow.
There is no hardcoded requirement that what is in the Summary be the final commit message, even if that may be convention.
You can make changes to that directly anyway, as our reviews are configured to allow All Users to edit them.
From my perspective, any comments you've got on the commit message can be made as general comments on the review with minimal impact - so nothing further is needed here.
I know Phab is not Gerrit. But Gerrit is an excellent review tool, so what is wrong in making use of the good features in it? It's super ignorant to say "I'm after a Gerrit centric workflow". I'm after a _good_ review workflow. Gerrit just happens to be one.
I need a way to review the final commit message. Newcomers have a problem writing good commit messages. How do I do that with phabricator, when I don't even _know_ what the final message will be? This is plain broken!