Store source permalink on XMLRPC calls

In a theme I’m developing, I need to link to the source post when the post has been stored using XMLRPC. That implies that every time WordPress asks for a permalink (using get_permalink()) the theme will return a previously saved link. That’s accomplished by adding a new filter:

add_action('the_permalink', 'filterPermalink');

function filterPermalink($url) {
    $permalink = get_previously_saved_permalink(get_the_ID());
    return $permalink? $permalink : $url;
}

That’s not the problem (by the moment). The problem is, how to make the same with the metaWeblog.newPost function.

I’ve already tried something like this:

add_filter('xmlrpc_methods','xml_rpc_functions_to_add');

function xml_rpc_functions_to_add($args) {
    $args['metaWeblog.newPost'] = 'add_permalink';
    return $args;
}

function add_permalink($args) {
    // Do it
}

What I need is to know how to call the previous ‘metaWeblog.newPost’ from ‘add_permalink’.