Please give it a try and tell me if it solves this error for you.

Error running :
SELECT DISTINCT MONTH(post_date) AS month, YEAR(post_date) AS year
FROM wp_posts
WHERE post_date > ‘2010-02-01′
AND MONTH( post_date ) != MONTH( ‘2010-02-01′ )
AND post_type = ‘post’ AND post_status = ‘publish’
ORDER BY post_date ASC
LIMIT 1
—- converted to —-
SELECT DISTINCT EXTRACT(MONTH FROM post_date) AS month, EXTRACT(YEAR FROM post_date) AS year
FROM wp_posts
WHERE post_date > ‘2010-02-01′
AND EXTRACT(MONTH FROM post_date ) != EXTRACT(MONTH FROM ‘2010-02-01′ )
AND post_type = ‘post’ AND post_status = ‘publish’
ORDER BY post_date ASC
LIMIT 1
—-
ERROR: function pg_catalog.date_part(unknown, unknown) is not unique at character 184
HINT: Could not choose a best candidate function. You might need to add explicit type casts.
———————

I don’t know much about the internals in Wordpress MU, but I think PG4WP may work with it.
If you try it, I’d love to get feedback about this (even if it doesn’t work, I think there would be little work to have it working).