Comment 8 for bug 188288

To be honest, this doesn't seem valid to me. It breaks the whole design of that structure, as it was intended to be opaque to the users of the API. It's a pretty common design pattern used throughout GNOME, specifically in this case to hide the NautilusMenuItemDetails from the consumers, forcing them to use nautilus_menu_item_set_[].

Iis there a reason nautilus_menu_item_set_submenu() won't work? Something like

NautilusMenu *nautilus_menu_mine = nautilus_menu_new ();
NautilusMenuItem *nautilus_menu_item_mine = nautilus_menu_item_new ("NautilusMenuMine::NautilusMenuMine", "My New Nautilus Menu Item", "Yaay", NULL);
nautilus_menu_item_set_submenu (nautilus_menu_item_mine, nautilus_menu_mine);
NautilusMenuItem *sub_menu_item = nautilus_menu_item_new ("NautilusMenuMine::SubMenuItem", "SubMenuItem", "Some More about it", NULL);
nautilus_menu_append_item (nautilus_menu_item, sub_menu_item);

That might be off a bit, but it's the general idea if I'm understanding the code correctly.