Replication chaining with PostgreSQL
You are right, PostgreSQL's built-in replication (aka Hot Standby) replicates whole cluster - so it's not suitable in your case.

You will need some trigger-based solution. For example,


Slony-I - most mature, written in C, flexible, good for complex setups
SkyTools pgq+londiste - C+Python, more lighweight than Slony-I
RubyRep - Ruby/JRuby, simple, easy to setup, not so mature


Please note that triggers always cause some extra load, and also initial sync will be equivalent of dumping all replicated tables.

Hope this helps, I'd propose to come back with more detailed questions if you have problems.

Good luck!