call to add_filter used with wordpress class

<?php
/**
* @package myplug
*/
/*
Plugin Name: myplug
Plugin URI: myplug
Description: myplug
Domain Path: /lang
Version: 0.1
Author: myplug
Author URI: myplug
License: GPLv2 or later
*/
if ( ! class_exists('myplug') )
{
    class myplug
    {
        public function __construct()
        {
            add_action('admin_menu', array(&$this, 'myplug_adminmenu'));
        }

        public function myplug_upload_dir_inside()
        {
            return array(
                'path' =>plugin_dir_path( __FILE__ ).'uploadtmp',
                'url' => plugin_dir_url( __FILE__ ).'uploadtmp',
                'subdir' => '',
                'basedir' =>plugin_dir_path( __FILE__ ).'uploadtmp',
                'baseurl' => plugin_dir_url( __FILE__ ).'uploadtmp',
                'error' => false
            );
        }

        public function myplug_menu()
        {
//Neither this call works
// add_filter('upload_dir', array(&$this, 'myplug_upload_dir_inside'));
//Nor this call either works !
// add_filter('upload_dir', 'myplug_upload_dir_outside');
            echo '<pre>'; print_r(wp_upload_dir()); echo '</pre>';
            wp_enqueue_script('plupload-handlers');
            $form_class = 'media-upload-form type-form validate';
            echo '<form enctype="multipart/form-data" method="post" action="'.admin_url("media-upload.php?inline=&amp;upload-page-form=").'" class="'.$form_class.'" id="file-form">';
            media_upload_form();
            echo '
            <script type="text/javascript">
            jQuery(function($){
                var preloaded = $(".media-item.preloaded");
                if ( preloaded.length > 0 ) {
                    preloaded.each(function(){prepareMediaItem({id:this.id.replace(/[^0-9]/g, "")},"");});
                }
                updateMediaForm();
                post_id = 0;
                shortform = 1;
            });
            </script>
            <input type="hidden" name="post_id" id="post_id" value="0" />';
            wp_nonce_field("media-form");
            echo '<div id="media-items" class="hide-if-no-js"></div>';
            submit_button( __( "Save all changes" ), "button savebutton hidden", "save" );
            echo '</form>';
            // remove_filter('upload_dir', 'manuland_upload_dir');
        }

        public function myplug_adminmenu()
        {
            add_menu_page('Myplugin', 'Myplugin', 'manage_options', 'myplug_menu', array(&$this, 'myplug_menu'));
        }
    }
}
$myplug = new myplug();
function myplug_upload_dir_outside()
{
    return array(
        'path' =>plugin_dir_path( __FILE__ ).'uploadtmp',
        'url' => plugin_dir_url( __FILE__ ).'uploadtmp',
        'subdir' => '',
        'basedir' =>plugin_dir_path( __FILE__ ).'uploadtmp',
        'baseurl' => plugin_dir_url( __FILE__ ).'uploadtmp',
        'error' => false
    );
}
//This call works though !
//add_filter('upload_dir', 'myplug_upload_dir_outside');
//BUT this filter can't be limited to this plugin : when i upload from library the upload dir is also filtered
?>