using system catalogs

Hi:

I'm definitely a postgres newbie, and was wondering about how much can be done with the system
catalogs. I'd like to a create table that links up with the pg_database catalog so I can track more
info on each database (users, associated application, status, etc).

- I assume I cannot and should not attempt to add fields to pg_database

- I have tried to create a table that uses has a reference to pg_database (datname) which it
complains about. ERROR:  permission denied: "pg_database" is a system catalog_database(datname));

- I can create another table with a field that matches the datname in pg_database and try to write a
trigger that inserts into my table when a record is inserted into pg_database, but I have the feeling
that may also be
a dead end.

On a related note, I'm also trying to find the system table that links database oids with table or
schema oids, but I can't seem to find it. I know it's there somewhere, but don't see it in the doc.

thanks!
Mija