No, I don’t think so. Maintenance mode is only useful for very limited use cases, because it requires a working Kirby installation. If that cannot be guaranteed, it is a better idea to use a separate html file.
Kirby will break and maintenance of no use, if parts of your site are missing.
Thanks for the hint, but I prefer to use static HTML / .htacess to take site offline. For me maintenance mode usually means up i’m deleting Kirby and uploading a fresh copy, so a plugin isn’t much good here because Kirby is missing.