PHP to copy Domain Access settings from one node to another (for FeedAPI)?

In the parent node object, there should be two elements:

$node->domain_site == BOOLEAN TRUE/FALSE
$node->domain == array() of active domains

The only trick is that $node->domain may have an element of -1, which is the default domain. We have to use -1 because FormsAPI checkboxes do not allow for a 0 to be passed as a value. So sometimes we have to convert the -1 to 0. You should not have to here.

See domain_node_access_records() to see how these values are handled. In that function, you can ignore the $node->domains_raw variable, which is only used for non-privileged users.

So you probably need something like this:

// Load the parent node.
$parent = node_load($nid_of_parent_node);

// Get the child node
$child = result_of_feed_api_function();

// Load the parent rules onto the child.
$child->domain_site = $parent->domain_site;
$child->domains = $parent->domains;

node_submit($child);
node_save($child);

That should be about it. node_save() will invoke the node access rules for you.