Would be nice, if the maintenance mode would work while update process is running.
If I upload the updates via FTP I just have a blank page - so, the maintenance mode did not work here.
That is because the system is doing "his thing". Maintenance mode is not designed for be used when updating the script, is when you need to backup or maybe change something in your server.
Maintenance mode runs over G\ and G\app so it just won't work when you are updating the files. The only thing that could work will be an static html or a plain text "maintenance" message.