Full (special) logs for specified users/hosts/etc

Hello hackers,

Consider a Postgres cluster containing several DBs (for example several
projects/sites). If one wants to optimize queries on one specified site --
what should he do? His obvious need is to switch full logging for the exact
database on, collect the logs and analyze them using pgFouine/pgBadger.

But currently, there is no way to specify log settings for one database /
one user / one host. There are only *global* settings in postgres.conf.

We experienced this problem in various project several times already.
Usually it is solved with initdb-ing separate cluster, what is annoying
and difficult if you have replication installed and working.

How difficult is to implement *local* logging settings per
database/user/host? Is it worth to work on for future versions?