All of the monitoring tools I mentioned have the ability to react to defined conditions by executing configured commands. You could f.ex launch a script that invokes a command on the slave using SSH.

mon might be interesting as it's written in Perl itself.

The other option might be to write just the part that sends commands to clients but use a monitoring package to do the regular tasks. As the control script runs on the same machine as the monitor, it could peek at the monitor's data and occasionally initiate actions on its own.

Makeshifts last the longest.