I haven't seen any one else reply. I don't know if you've gotten a solution. But the following seemed to work for me:
WITH serie AS (
select s, s*10 as computing
from generate_series(1,10) as s
)
INSERT INTO test_insert_returning (some_value)
SELECT computing
FROM serie
RETURNING gid, some_value;
From my reading on the RETURNING phrase, you can only return values from the table into which you are doing the INSERT. Not any other table or view which might be referenced.