Finalmente google ha rilasciato le API per  Analytics.

Di seguito un semplice tutorial per potersi autenticare e utilizzarle:

$data = array(
    'accountType' => 'GOOGLE',
    'Email' => $email,
    'Passwd' => $password,
    'service' => 'analytics',
    'source' => ''
);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://www.google.com/accounts/ClientLogin");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
$output = curl_exec($ch);
$info = curl_getinfo($ch);
curl_close($ch);

$auth = '';
if($info['http_code'] == 200) {
    preg_match('/Auth=(.*)/', $output, $matches);
    if(isset($matches[1])) {
        $auth = $matches[1];
    }

Se il login ha succeso sara restituito lo status code 200, a questo punto si potrà accedere alle API con:

$headers = array("Authorization: GoogleLogin auth=$auth");
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);