Changing a node's domain access programatically

I've been scouring through the API docs and issues trying to find how to do this.

I found this http://drupal.org/node/316509#comment-1041902 as a clue, but either I'm doing it wrong or it doesn't work with D6.

I've been trying something along the lines of this:

$node = node_load($nid);

$node->domain_site = TRUE;
$node->domains = array(3, 4);

node_submit($node);
node_save($node);

Someone please tell me I've missed something obvious.