Mechanism for alerting on empty folders

It would be nice if you could setup an event rule, folder monitor or scheduled task to alert on an empty folder.

Use Case:

Cerberus server is being used as a configuration backup repository. System X stops sending backup files to the server and through the use of a folder monitor all the existing backups are aged out. Backup folder is now empty and there is no way to detect that backups are not working if the system sending them does not have alerting available.

This could also be used as a trigger mechanism to detect and remove stale FTP user accounts.