I think the GtkRadiant just sucks. The biggest advantage of the DoomRadiant is that it is integrated tightly into the engine and you have a good preview about the final results.
Actually porting the DoomRadiant would be quite cool. It was some of my first experiments with the Doom 3 GPL code.
Ensiform created new GUIs for the main window and several inspectors however it was designed as a standalone app.
I got it to work inside of D3 and wrapped D3 into a Qt widget so it can be docked into the QtRadiant.
If I was doing it again I would scrap all written code and use the Qt windows migration framework and starting to replace individual inspectors.
The main problem is that it is a lot of work to finish it and it requires someone who steps up on the plate and is really dedicated for more than 2 weeks.
I'm really busy getting my stuff done so I simply live with the MFC tools that I improved with some smaller tweaks like Sikkpin did with WASD camera controls and other stuff.
I think it is not a good idea to make a radiant just in Qt. If you want to port the entire thing then I suggest to have a look at the Unreal 4 editor.
That's the stuff people are really looking for ... especially the content browser.
Creating a level with drag 'n' drop from the models you created in Blender simply rules. It would be also nice to create prefabs with a preview picture and add tags to them like mp3 tags for better searching.
UE4 also offers a nice workflow by exporting the whitebox game prototype geometry as .obj.
The content browser is really a productivity booster.