I know there are many GUI toolkits around for OpenGL. I'm looking for a GPL or LGPL toolkit that can -render- dialogs and controls with both OpenGL and DirectX. Is such thing available ? I had QT in mind, is it an alternative for that task ? I don't know about QT integration on a DX surface...

http://crayzedsgui.sourceforge.net/

CrazyEddie's GUI system is indeed moving to become Cross-platform. At last word, there was OGRE rendering support along with DirectX.

Failing that, you could take a look at the fine tutorials around here and build them up onto an abstracted rendering system. There's tutorials for DirectX GUIs and OpenGL GUIs around. I'm sure you could quite easily separate the rendering from the GUI handling and write pluggable renderers.