It won’t work because review.py use shell commands to get the information from mercurial whereas hgreview directly plugs into mercurial internals. Unfortunately the function hgreview is using to get the modified / added / deleted files works can be executed on subrepos but do not return this data (Mercurial: 87bfd1703597 mercurial/context.py).
Of course using itersubrepos is an option I plan to implement but not right now.