Thanks to a patch Dane Miller submitted, MySQL Heartbeat can now be used to monitor Slony-I replication lag for PostgreSQL. It works identically: it updates a record on the master and looks for it on the slave. You just have to specify the database driver on the command-line:
mysql-heartbeat [options] --dbidriver=Pg
Most of the tools in the MySQL Toolkit are very MySQL-specific. This one just happened to be an exception, and I'm happy it's useful for more than the original purpose.