Putting extra Tempcode into the templates for that exceptional situation would create more problems that it solved (by confusing regular themers with extra complexity).

Anyone can add some Tempcode to check if the site is closed if they wish...
{+START,IF,{$CONFIG_OPTION,site_closed}}...{+END}

Or you can manually upload a closed.html (IIRC) file which shows instead of the regular Composr closed site message. If you really want robustness then that's a better idea, as it doesn't then require Composr's database to be initialised and config options loaded etc.

closed.html sounds perfect, I didn't even know that was an option. Thank you.