Our custom WPF-control with designer for Visual Studio 2010 used to work by adding the corresponding Registry Keys under:
HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\10.0\DesignerPlatforms
(we used the technique described in Karl Shifflett's article on Extensibility Series – Loading Metadata for Microsoft Controls)
I noticed that the registry organisation for Visual Studio 2012 has changed - which are now the corresponding keys, please?