Programmatically insert webform result
Have you tried using PHP to manually send the POST data? Here is a snippet from http://www.webmasterworld.com/php/3164561.htm

$server= 'www.someserver.com';
$url = '/path/to/webform/posturl';
$content = 'name1=value1&amp;name2=value2';
$content_length = strlen($content);
$headers= "POST $url HTTP/1.0\r\nContent-type: text/html\r\nHost: $server\r\nContent-length: $content_length\r\n\r\n";
$fp = fsockopen($server, $port, $errno, $errstr);
if (!$fp) {
    return false;
}
fputs($fp, $headers);
fputs($fp, $content);
$ret = "";
while (!feof($fp)) {
    $ret .= fgets($fp, 1024);
}
fclose($fp);
print $ret;


Of course this would fail if you are using any form of captcha on the form, and will also currently fail if you are required to be authenticated to post a reponse