That specific MongoDB feature is "tailing the oplog", where the oplog is the capped collection with the idempotent commands that represent the changes in the database. This is what Meteor uses to receive (be pushed) changes (not polling).
Certainly, logical decoding provides the necessary infrastructure to implement it (and also normal tailable cursors): thank you Andres, really nice work! :) But some work is also needed to transform the representation you are using in PostgreSQL into MongoDB's oplog entries.
This is definitely what we are using in ToroDB to emulate it (currently, under development).
http://www.postgresql.org/docs/devel/static/logicaldecoding.... for the (somewhat low level) description of the feature. You'd have to write an output plugin that formats the output as json, but that should be pretty easy.
Disclaimer: I'm the author of the feature ;)