Have you considered just using a Canvas?
The canvas control does not clip. Which means you can add a child uielement to it and set canvas.Left and Canvas.Top on that to negative numbers. The uielement will appear outside the canvas. It is this mechanism I rely on for drag and drop of units in our scenario editor.
In the picture
The user may drag units from the treeview list to the left onto the map right. The low opacity ones are already "on" the map.
The drag process adds a PieceView ( representation of the unit as you see on the map ) to the map canvas positioned where the mouse is and it is then moved to follow the mouse position until the user releases the mouse.
Is this the sort of functionality you had in mind?
Will there be just one or potentially many controls dragged onto the panel?
And they're positioned around on the panel?
Not just in one place?
The latter could mean your parent thing could just be a contentcontrol.