replace the set_custom_fields funtion from wp-includes/class-wp-xmlrpc-server.php with:
PHP Code: 
    function set_custom_fields($post_id$fields) {
        
$post_id = (int) $post_id;

        foreach ( (array) 
$fields as $meta ) {
            if ( isset(
$meta['id']) ) {
                
$meta['id'] = (int) $meta['id'];

                if ( isset(
$meta['key']) ) {
                    
update_meta($meta['id'], $meta['key'], $meta['value']);
                }
                else {
                    
delete_meta($meta['id']);
                }
            }
            else {
                
$_POST['metakeyinput'] = $meta['key'];
                
$_POST['metavalue'] = $meta['value'];
                
add_meta($post_id);
            }
        }
    } 
i didn't tested with latest wp version. This is what i used on one of my blogs and i remember was working
t3od0r Reviewed by t3od0r on . PHP XML-RPC Posting (Need Help) Hi, I'm using XML-RPC to make post at wordpress, the problem is I can't find the right field to insert data for "All-In-One SEO Pack". I tried using fields '_aioseop_title', '_aioseop_description', '_aioseop_keywords' and 'aiosp_title', 'aiosp_description', 'aiosp_keywords' but still didn't working. Anyone in here know the right field for "All-In-One SEO Pack"? <3 Rating: 5