I've got something very similar to that working now that doesn't break the DBIx::Class style too much. The SQL is (roughly)

SELECT i.item_id FROM items AS i LEFT JOIN mapping AS m ON (m.item_id 
+= i.item_id)
GROUP BY i.item_id HAVING EVERY(m.container_id != 1) OR EVERY(m.contai
+ner_id != 1) IS NULL
[download]

I'm not sure of a couple things: whether this is better performance-wise than what you posted or whether EVERY (or equivalent BOOL_AND) is portable beyond PostgreSQL. However, it does translate easily into DBIx::Class:

my $rs = $schema->resultset('items');
my $items = $rs->search(undef, {
  join => 'mapping',
  group_by => [ map { "me.$_" } $rs->result_source->columns ],
  having => "EVERY(m.container_id != 1) OR EVERY(m.container_id != 1) 
+IS NULL",
});
[download]

The map feels a little hackish, and I don't get to use a placeholder for the container_id, but overall, I'm not too unhappy with it.

Update: Added the additional "IS NULL" clause, necessary for items which are not contained in any container.