Interesting, then as a workaround trackers may create an image with pseudo-random src pointing to pageXXXwebsiteYYYclientZZZ.ad.yoursite.com (yoursite.com is the site which serves the actual content), while asking the owner to point NS record for ad.yoursite.com to IP of their DNS server. So HTTP/S request or DNS request can reach them anyway. Obviously DNS caching will prevent them from knowing how many times this particular page was accessed by this particular client, but at least they will know that it was accessed at least once.