create main menu-highlight current page even when on child

I am trying to use wp_list_pages (or not, I’m flexible) to create a main menu that will output only some of my top level pages, not their children. No problem there.

But, I want some class to be output (.current_page_item for example) on one of those parents when I am on a child of that parent.

For example, when I am on “photos > my favorites” I want my top level page “photos” menu item to be identified as the active page.

Is this possible?

Thanks much, you’re a peach!