I answered my own question using built-in JSON functions.

SELECT n, ARRAY(SELECT JSONB_ARRAY_ELEMENTS(JSONB_ARRAY_ELEMENTS(JSONB_AGG(a)))) AS a, SUM(q) AS q
FROM (
    SELECT 'foo' AS n, '{1,2}'::integer[] AS a, 1 AS q
    UNION ALL
    SELECT 'foo' AS n, '{3,4,5}'::integer[] AS a, 1 AS q
) results
GROUP BY n