Использовать XMLRPC. Примерно так:
$params = array(0,$username,$password,$content,true);
$request = xmlrpc_encode_request('metaWeblog.newPost',$params);
$ch = curl_init();
curl_setopt($ch, CURLOPT_POSTFIELDS, $request);
curl_setopt($ch, CURLOPT_URL, $rpcurl);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 1);
$results = curl_exec($ch);
curl_close($ch);