Is there a way, in a single SQL statement to ensure that all items in a list are present in some column?
If I understand what you're trying to achieve, then I think what you want is a CTE with a UNION list of all the possible tags you want to compare against, then RIGHT JOIN  the tag table against the CTE.

Example -

WITH ListOfTags (Tag) AS (SELECT 'Foo' UNION SELECT 'bar' UNION SELECT 'other' UNION SELECT 'thing')
SELECT  *
FROM    item_tag    i
INNER JOIN  tags    t
    ON i.tag_id = t.tag_id
RIGHT JOIN ListOfTags   l
    ON  l.Tag like t.tag
WHERE   i.item_id is null


This might not be the most elegant way of doing it, but it might work for you