I understand that:
1) you like to use postgres as a "bus" to transfer messages between connected clients;
2) only one database server is concerned (no redundancy at all);
3) it is the client code (perl, php ...) that send the notification (ie, notifications are not sent by triggers for example)
May be you could dedicate one of your database to do this; all clients could listen messages on this database and some client code would decode the payload and does its job.
Sylvain