If the maintenance reason is a url, then redirect to this address

mod/maintenance.php

@@ -3,12 +3,21 @@
 use \Friendica\Core\Config;
 
 function maintenance_content(App $a) {
+
+	$reason = Config::get('system', 'maintenance_reason');
+
+	if (substr(normalise_link($reason), 0, 7) == 'http://') {
+		header("HTTP/1.1 307 Temporary Redirect");
+		header("Location:".$reason);
+		return;
+	}
+
 	header('HTTP/1.1 503 Service Temporarily Unavailable');
 	header('Status: 503 Service Temporarily Unavailable');
 	header('Retry-After: 600');
 
 	return replace_macros(get_markup_template('maintenance.tpl'), array(
 		'$sysdown' => t('System down for maintenance'),
-		'$reason' => Config::get('system', 'maintenance_reason')
+		'$reason' => $reason
 	));
 }

util/maintenance.php

@@ -49,5 +49,5 @@ if ($reason != '') {
 }
 
 echo "Usage:\n\n";
-echo "\tphp {$argv[0]} [1] [Maintenance reason]\tSet the system in maintenance mode\n";
+echo "\tphp {$argv[0]} [1] [Maintenance reason|redirection url]\tSet the system in maintenance mode\n";
 echo "\tphp {$argv[0]} 0  \tSet the system in normal mode\n\n";