I wonder if it would be feasible to add this sort of real-time feed to a conventional RDBMS like PostgreSQL. This feature would be quite useful for at least one of my projects, but I'm not sure I want to give up a mature SQL-based DBMS for something relatively unproven like RethinkDB.
You could probably get a poor's man version of this by setting up materialized views, a trigger on those views, and then write code in the trigger that pushes updates onto a queueing system. It wouldn't be quite as convenient, but would probably accomplish what you want in many situations.