Walker_Nav_Menu doesn’t work in wp_page_menu_args filter

I need to hide certain nav items from the menu (based on the password protection functionality) and I’m having trouble getting my custom Walker to work. In fact, I can’t even get the standard Walker_Nav_Menu to work when I add it via a filter.

I’ve created a plugin as follows:

<?php
function myplugin_page_menu_args( $args ) {
    $args['walker'] = new Walker_Nav_Menu;
    return $args;
}

add_filter( 'wp_page_menu_args', 'myplugin_page_menu_args' );
?>

There’s no other code in the plugin file, just those 6 lines. I’ve added this plugin to a brand new install of WordPress (http://dev.matt-howe.co.uk/wordpress/) and the menu rendering is completely broken – all the items are on the same level, and none of them have any text. The right number of <li> elements are generated, and the ids are correct, but the rest is wrong.

I’ve tried using a custom class with a .walk() method, as so:

class myplugin_walker {
    function walk ( $elements, $to_depth ) {
        return 'test';
    }
}

function myplugin_page_menu_args( $args ) {
    $args['walker'] = new myplugin_walker;
    return $args;
}

add_filter( 'wp_page_menu_args', 'myplugin_page_menu_args' );

This works exactly as expected – the menu renders as ‘test’. But trying to use either my own class which extends Walker_Nav_Menu, or the core Walker_Nav_Menu class itself, results in a flat list of empty <li> tags.

I’ve tried a few other ways to achieve the same result – I’ve tried adding filters for wp_nav_menu_objects and wp_get_nav_menu_items but neither of my functions seem to get called at all.

Am I barking up the wrong tree here – is this something that won’t work as part of a plugin, and needs to be elsewhere in the code? Or am I just missing something obvious?