I'd rather suggest you putting the command in the nagios.cmd file. Then you can put the external command for scheduling downtime from the script .
Have you enabled check_external_command option in nagios.cfg and apply write rights to the nagios.cmd file?
List of external commands:
nagios.org/developerinfo/ext ... ndlist.php
Another thing you could do. You say that it is a fixed downtime for those servers. Define a new timeperiod which excludes the period of downtime and assign it to those servers.