First off, I am aware of Mercurial: Problem with non-ascii letters in filenames between Windows and Linux and What DVCS support Unicode filenames?. But they are old, and do not apply to the latest versions of mercurial.
They both mention the FixUtf8 extension - which does not work with the latest version of mercurial (2.3.1 as of this writing) and apparently has not worked since mercurial 2.0. In fact enabling that extension will prevent you from adding any files at all, even plain "vanilla" ascii filenames. The FixUtf8Extension appears to have been abandoned - it's last commit was in 2010.
I have been a happy mercurial user up until now, but I am now on an international project where support for non-ascii filenames is critical. I will have to look at alternate version control systems if this can't be addressed in mercurial.
Is there any support for unicode filenames at all with the latest versions of mercurial on Windows?