Can you really produce a wordpress plugin that, when active, would give a new "function" towards the XMLRPC interface and take care of its calling?

In a nutshell, yes. You can include a work as whether plug-in or perhaps in your theme's functions.php file that handles XMLRPC calls. You will need the next sections:

function xml_add_method( $methods ) {
    $methods['myClient.myMethod'] = 'my_method_callback';
    return $methods;
}

add_filter( 'xmlrpc_methods', 'xml_add_method');

This function adds your method call towards the built-in XMLRPC method handler. When someone constitutes a request to http://yoursite.com/xmlrpc.php with this particular method, all parameters will be delivered to the my_method_callback() function:

function my_method_callback( $args ) {
    // Do Something

    // Return Something
}

I personally use this technique to deal with error confirming with my plug-inches. When among my plug-inches malfunctions on the client's website, it reviews the malfunction by posting data to http://www.mywordpressinstallation.com/xmlrpc.php. On my small site, I've got a plug-for the reason that stores these details inside a database in order to evaluate it later and connect bugs.