Lately I've been working on a connection pooler which only supports LISTEN / NOTIFY. The idea is to be able to keep the number of Postgres connections down without having to give up (or come up with a workaround for) notifications. With allas you can e.g. use pgbouncer or your environment's native connection pool and open a separate connection for notifications only. allas internally uses only a single connection to PostgreSQL, so you can keep your max_connections setting down at a reasonable level and get more out of your database server.
If this sounds interesting, check it out!