Drag-n-drop between tabs

[saulgoode flashingtwelve brickfilms com]

I am interested in enhancing the drag-n-drop functionality of the GIMP  
and am unable to find whether GTK+ supports the feature I desire.

The GIMP uses a notebook widget for docking various windows and it is  
often convenient to have the Layers window and Channels present in the  
same notebook (as shown in the first figure in  
http://docs.gimp.org/en/gimp-concepts-docks.html). The problem I wish  
to solve is being able to drag a layer from the Layers tab over  to  
the Channels tab. This is currently only possible (to my knowledge) if  
I detach one of the tabs and thus far I have been unable to find how  
this would be implemented.

If there is a way to do this, I would appreciate if someone could  
point me in the right direction. If not, would it be feasible to  
enhance the notebook widget so that hovering the pointer over a tab  
while a mouse button is depressed for a certain duration (one or two  
seconds) would raise that tab?