Using the Mango Persistent TCP data source and publisher is pretty ideal for this. There are a number of things you can publish to a central Mango for monitoring such as a random or incrementing value. Also the Internal data source values such as JVM memory usage, and task lists. There is also a little known data source called the Log4J Data Source which allows you to turn system log messages into Alphanumeric data points. You can have an Error level and a Warning level data point publishing to the monitoring Mango. You can then have a change event detector on this so if there are any Errors that show up in the log file they will be sent to the main server and you will be notified.
It's also our longer term plan to offer a hosted solution specifically for doing this monitoring and alarming and we'll probably package these features up into a simple to install and configure module so you don't need to do much for set up.