I'm aware of this problem and as far as I can tell, it's impossible to fix in just CSS, as the HTML structure of these comments is exactly the same as the "merge" comments – only the text differs.

I would personally say 'no' (although I obviously filed the task, lol). IMO this is a UX issue that should be solved if possible.
As to how this might be solved, I've had some ideas about potentially adding some custom WMF/downstream code in the PHP file(s) that parse Remarkup, to potentially see if some extra/custom HTML classes could be added there that might then be able to be picked up by the new gerritbot CSS. I haven't yet had a chance to properly investigate that possibility, but IMO it feels like it might be a possibility.