Conditional add_filter for upload directory?

I have a custom media upload button (in the standard post edit page, alongside the standard media upload button), and need to change the upload directory dynamically if this button is clicked – using add_filter(‘wp_handle_upload_prefilter’, ‘my_function’).

This post & answer: Conditional add_filter? seem to give the solution, but for some reason don’t work for me – the additional parameter ‘hexProtected’ is not set & so add_filter is not called.

My code is below… am I doing something stupid or does the solution posted not work?!

<?php
/* Custom Upload Directory for protected posts */                                    

function hex_protected_media_button($context) {
    global $post;
    $media_button_image = 'http://www.example.com/wp-admin/images/media-button.png?ver=20111005';
    $media_button = ' %s' . '<a href="media-upload.php?post_id='.$post->ID.'&amp;TB_iframe=1&amp;hexProtected=1" class="thickbox"><img src="'.$media_button_image.'" /></a>';
    return sprintf($context, $media_button);
}

add_filter('media_buttons_context', 'hex_protected_media_button');                                                           
add_filter('wp_handle_upload_prefilter', 'hex_pre_upload');
add_filter('wp_handle_upload', 'hex_post_upload');

function hex_pre_upload($file){  
    if (isset($_GET['hexProtected'])) {
       add_filter('upload_dir', 'hex_custom_upload_dir');
    }
    return $file;
}

function hex_custom_upload_dir($path){      
    if(!empty($path['error'])) { return $path; } //error; do nothing.   
    $path['path'] = str_replace($path['subdir'], '/protected'.$path['subdir'], $path['path']);
    $path['url'] = str_replace($path['subdir'], '/protected'.$path['subdir'], $path['url']);        
    $path['subdir']  = '/protected';    
    return $path;
}

?>