I've written a shell script that will do something just like this. What it does is check the MAC address of your current router. If that MAC address has changed then it will force dismount any AFP mounted disks and maybe any SSHFS mounted disks. I have this script run every minute via a launchd process that I set up with Lingon. The script will log its activity if you set it up that way.
Here's the script. If you have any problems let me know. I've been running it here for over a month without problems.
I've name the script 'unmount.sh' Let me know what you think. If you have problems with it.
Andy