Duplicate child menu items – how to preserve visitor path

I have a client who’s created a Site Map that has duplicate references to the same page.

For instance, the Contact page is a child of every Level 1 page.

She wants a contextual sidebar menu to reflect the path the visitor has taken to get to the page – so if they click the link to Contact under the About Us (Level 1) menu, then the sidebar should show the About menu and children.

If they visit the Contact page by clicking the link under News, then the News menu should remain in the sidebar. Ideally, the path itself should reflect this as well (ie, /news/contact/)

I can’t do this using native Page hierarchy, because each page can only have one (or zero) parents. So I’ve left all pages without parents, and created a custom Menu to reflect the site map she wants. But I can’t find a way to display the proper menu.