I'm aware of the psql command that will list these, but I'm attempting to write an extension for Oracle's SQL Developer that will list them on the left-hand navigator panel. The XML format requires a select statement.
For example purposes, I'll include the code I cooked up for sequences:
<sql constrained="true">
<![CDATA[SELECT relname FROM pg_class JOIN pg_namespace ON pg_namespace.oid = pg_class.relnamespace WHERE relkind = 'S' AND nspname = :SCHEMA]]>
</sql>
I've attempted to figure it out for myself, but I don't see anything with my test domain names in pg_class or any other of those internal tables (though it must be in one, somewhere).
Does anyone have a hint?
Asked too soon... here's a select query that will return domains from a specific schema:
SELECT typname FROM pg_catalog.pg_type JOIN pg_catalog.pg_namespace ON pg_namespace.oid = pg_type.typnamespace WHERE typtype = 'd' AND nspname = 'someschema'
Minor tweaking can also return enums, ranges, and composites/rows. More information in the docs.