Im not sure how to do this with gdk but you could just use xlib
Here is a tutorial on having a window move itself around the screen etc.
http://users.actcom.co.il/~choo/lupg/tu ... w_ops_move
I don't think libwnck is going to help you out much from what i know of it its only purpose is for building tasklists and desktop pagers.
Another way to possibly move a window around that has no decoration is to just
alt + spacebar
or ctrl + spacebar but this will only work if your window manager supports it.
So this is what i would do.
When you click on the window that has no frame add in some xlib code to get the window "id"
Then assign a xlib mouse move event to have the window follow your mouse cursor.