Has anyone had any luck augmenting DirectGUI with drag & drop support?
Yes, I believe a few different people have done such a thing over the years. Try searching the forums.
Sorry for being imprecise/incomplete.
Can you point me to the branch in svn that contains the code that implements this?
Specifically, I’m looking for a good implementation of drag & drop to/from DirectGUI 2D widgets and the 3D scene graph.
If you want such function, you will have to search the forum for snippets that implement it. Then you will probably have to re-code it to fit your particular needs. Such code is not part of Panda, not in the official releases nor in the development branches.