Custom links per domain

You're lucky I'm nice, while it probably would have been helpful had I shared the information straight away, finding the specifics from so long ago, especially when the project is long completed and the only copy I have is in a Time Machine backup of an old computer, was quite tricky... but find it I did.

This is only conceptual code, as I won't share a clients code without there permission, nor would some of the code even be of any use to you.

First thing I did was inside hook_cron() I would check if the custom links where already in the xmlsitemap table, as it was an ever growing list it was more than likely that there would be new items to build, in that case I would build up the link object and trigger it to be saved with code like the following:

$link = array(
  'type' => 'custom',
  'id' => db_query("SELECT MAX(id) FROM {xmlsitemap} WHERE type = 'custom'")->fetchField() + 1,
  'loc' => '[path to link]',
  'priority' => '0.5',
  'language' => 'und',
);
xmlsitemap_link_save($link);

Then comes the tricky bit, the query alter. It's tricky to really walk you through this one as it was done so long ago and I have no desire to get my head to deep in to it as I have no need to do so, so I'll just dump the slightly modified code and let you work it out for yourself:

function MYMODULE_query_xmlsitemap_generate_alter(QueryAlterableInterface $query) {
  $sitemap = $query->getMetaData('sitemap');
  if (!isset($sitemap->context['domain'])) {
    return;
  }

  // Showroom permissions check.
  $domain_condition = db_or();
  $array = explode(',', SOME_DOMAIN_SPECIFIC_CALLBACK());
  foreach ($array as $item) {
    $domain_condition->condition('loc', 'URL/' . $item);
  }
  $query->condition(
    db_or()->condition(
      db_and()->condition('loc', 'URL/%', 'LIKE')
        ->condition($domain_condition)
    )
    ->condition('loc', 'URL/%', 'NOT LIKE')
  );
}

Hope it helps