Ejecting users from server before automatic shutdown

I have a MM functioning as a blind server. It is, among other things, used by Time Machine (other Macs on the network backup to it) as well as ChronoSync (other macs on the network backup to it). The network is hardwired > 10BaseT.

Of course from time to time a volume is still mounted on one of the Macs that backup to the MM at MM shutdown time.

The server powers down (via energy saver) every evening it than hangs in that case and does not turn of. I have solved it with a crude Cron script that kicks in 10 minutes after the scheduled shutdown, but this is not the way to go.

So what I am after is a way to disconnect any network Macs that are connected with the MM before the official shutdown happens. My preference would be to this at the MM end and not on the other computers. I am thinking here of Apple Scripts or Cron commands etc.

Any suggestions out there?