How to alias attributes in an ARRAY_AGG expression

Hello,
I'm playing with this query, my goal is return an entire json response with
some column names changed in the resultset of ARRAY_AGG::

SELECT ARRAY_TO_JSON(ARRAY_AGG(ALBUM_ROW))
FROM (
SELECT
album,
MAX(release_year) AS release_year,
MAX(artwork_path) AS artwork_path,
MAX(MD5(CONCAT(album, release_year, artist))) AS token,
ARRAY_AGG((media_files.position, media_files.token) ORDER BY
media_files.position) as media_files
FROM media_files
INNER JOIN playlist_media_files ON playlist_media_files.media_file_id =
media_files.id
WHERE playlist_media_files.playlist_id = 1
GROUP BY album, release_year, artist
ORDER BY artist, release_year
) as ALBUM_ROW

This works fairly well, however I need to alias the attribute names in the
ARRAY_AGG:
ARRAY_AGG((media_files.position, media_files.token) ...

ARRAY_AGG apparently does not support AS, so I have to resort to a
subquery, which works but is totally inefficient:

SELECT ARRAY_TO_JSON(ARRAY_AGG(ALBUM_ROW))
FROM (
SELECT
album,
MAX(release_year) AS release_year,
MAX(artwork_path) AS artwork_path,
MAX(MD5(CONCAT(album, release_year, artist))) AS token,
(
SELECT ARRAY_AGG(d)
FROM (
SELECT mf.position AS myalias
FROM media_files AS mf
INNER JOIN playlist_media_files ON
playlist_media_files.media_file_id = mf.id
WHERE playlist_media_files.playlist_id = #{playlist_id}
AND mf.album = media_files.album
ORDER BY mf."position" ASC
) d
) as media_files
FROM media_files
INNER JOIN playlist_media_files ON
playlist_media_files.media_file_id = media_files.id
WHERE playlist_media_files.playlist_id = #{playlist_id}
GROUP BY album, release_year, artist
ORDER BY artist, release_year
) as ALBUM_ROW

Does anyone have better ideas?
Thanks,
C