The standard Index card is located z:index.wml. This should allow it to dig into the standard menu, but I'm not sure how your user would get back to the custom menu other than at startup as the standard menu has no links to go back from there.
Does this make sense?
If you just want to get rid of a custom menu, delete the wml files from your E: drive. You can temporarily override custom menus by pressing OK and Cancel while powering on.
Hope this helps!