How can I filter databases by owner using psql?
For things like this, you'll want to query pg_database directly. The owner field is a reference to pg_catalog.pg_authid.oid, but this is a superuser-only accessible table. There's a view, pg_catalog.pg_roles that exposes the id as the oid attribute, so you can:

select 
  datname,
  rolname AS dbowner
from pg_database d 
     inner join pg_roles r on (d.datdba = r.oid);