It would be useful to developers, specifically in designing on rare occasions to enforce character limits on translations.
See:
https://www.mail-archive.com/design@lists.wikimedia.org/msg00132.html
I'm not sure what is the best way to do this, but a magic word in a qqq code might be a useful easy way to do this.
When translating, if a message goes over this limit, the translator should be warned that this is the case (note I don't think it should stop the translation but it should certainly make them aware).
e.g.
{{ENFORCE_LIMIT=100}}