DND questions

[The Surprises <thesurprises1 comcast net>]

Hello,
I have a set of gnome canvases, each gnome canvas has a set of items.  I
already have implemented a drag routine for moving items around it's
parent canvas, but I would like to extend this to allow dragging across
canvases.  DnD will hopefully  do what I want (assuming the Canvas widget
accepts drag events?).  However, I want to use my drag routines until
the mouse pointer leaves the current canvas widget (with a leave_notify
event).  So the chain of events looks like this:

1.  User holds down button 1 on an item to begin a move
2.  User moves the item around it's parent canvas using my drag routines
3.  User leaves the canvas widget, signalling DnD to take over
4.  User releases the item in another canvas, and I use the DnD signals
    to send the data over.

I don't know how to do #3.  Is it possible to override when a DnD
starts?

Thanks
Jason