As you may know, when you interoperate WPF with other technologies you can't overlay WPF and WinForms/Win32 content because of the "airspace" limitations (read more here).
Any given pixel in a window is either owned by WPF or WinForms/Win32, not both. On the other hand, you can overlay all the WPF content you want because WPF manages and renders all of them.
Many folks however are looking to overlay WPF content over Web content. Chris Cavanagh posted sometimes ago a blog that explain how you can do so using Google's web control.
Sources for a sample are available on Codeplex. You would need to license the wrapper called Awesomium.