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

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?