I've got a set of admin pages which I've dropped into the wp-admin folder and I have a plugin in mu-plugins which defines the menu structure using
add_menu_page('CanalPlan Options: General Options', 'CanalPlan Options', 8,'cp-admin-menu.php');
add_submenu_page('cp-admin-menu.php', 'CanalPlan Options: General Options', 'General Options',8, 'cp-admin-general.php');
add_submenu_page('cp-admin-menu.php', 'CanalPlan Options: Home Mooring', 'Home Mooring', 8, 'cp-admin-home.php');
add_submenu_page('cp-admin-menu.php', 'CanalPlan Options: Favourites', 'Favourites', 8, 'cp-admin-fav.php');
And in 2.7 I get a little block at the bottom of the menu which expands to show all my sub menus and its working fine.
The admin bar shows the same menu but the links dont work as they are in the format:
/wp-admin/admin.php?page=cp-admin-general.php
The reason for using separate files is that each of the pages is quite complicated and I want to be able to maintain the code easily.