How does the show_message/display_message key value replacement function operate?

I've been trying the following:

$rcmail->output->command( 'display_message', $this->gettext('offline'), 'error', array('reason' => 'maintenance' ));

with the localization string:

$labels['offline'] = "Webmail is currently unavailable due to $reason.";

How do I link the replacement value from my localization string and the call to display_message?