I actually implemented this (i.e, your parent's idea) for a product. Took me an afternoon to read the documentation and source of the logical output plugin and build a logical replication receiver (using the guts of an existing binary-protocol driver).

Of course, that is a testament to the quality of the documentation and the source code of PostgreSQL.

The fact that the Logical Decoding and Logical Output features were built for replication means that some decisions were taken not-in-favour of publishing changes, obviously, but it works pleasantly and straightforwardly. Even writing a custom logical output plugin would be simple, because Pg does all the transaction tracking, position tracking, change decoding etc.

That sounds like you'd need to manually enumerate all possible queries you'd want to listen to and have all writers do the work of deciding which channels to update.

The idea of simply being able to listen to a query without any additional work sounds like it could be really attractive in certain situations.