This might be a little off the subject but having worked with
pl/sql and ref cursors ( of which I am really hoping will be
obsolete in the future ) I can recommend this article

http://www.oracle-base.com/articles/8i/ComplexRecordsets.php#PLSQLTables

for working with in memory pl/sql types and refcursors

create or replace
function get_rss_items
return sys_refcursor
as

l_cursor sys_refcursor;
begin
open l_cursor
for select *
from table( get_rss_items_tbl )
;
return l_cursor;
end;

Did you actually test this? I tried to do exactly what you propose in PL/SQL and the Oracle compiler gives me the error messagses:
PLS-00231 function ‘xxx’ may not be used in SQL

CREATE OR REPLACE TYPE sample_type_internal AS OBJECT(
phoneNumber number
);

CREATE OR REPLACE TYPE list_sample_type_internal IS TABLE OF sample_type_internal;

CREATE OR REPLACE TYPE sample_type AS OBJECT(
name VARCHAR2(20),
listPhoneNumbers list_sample_type_internal
);