Add menu item to edit specific page

You’ve almost answered your own question. Just pass the URL as the $menu_slug to add_menu_page:

add_menu_page(
    null, // not an actual page, so title is irrelevant
    'Menu Item Name',
    'edit_posts', // or whatever capability required for this object
    '/post.php?post=42&action=edit',
    null,
    '',
    6
);

The caveat is that when you visit this menu item, the actual menu item it belongs to will be highlighted in the menu (posts, pages, media). You could fix this with a bit of JS, or just live with it.