From look at source metaWeblog.newPost seems to be processed in wp_xmlrpc_server->mw_newPost() method.
At the end of this method there is following hook call:
do_action( 'xmlrpc_call_success_mw_newPost', $post_ID, $args );
which seems to be very fitting to process and save any additional information for post that have just been created by its ID provided.