OK, one Problem with trackback in WP is that someone seems to have forgotten that some blogs do need to receive a query-Part in the trackbacked URI. I have changed one line in wp-includes/functions.php:
was: $http_request = ‘POST ‘.$trackback_url[‘path’].” HTTP/1.0\r\n”;
is: $http_request = ‘POST ‘.$trackback_url[‘path’].”?”.$trackback_url[‘query’].” HTTP/1.0\r\n”;
and have now sucessfully pinged a pivot-powered blog.