Well, there are a lot of different ways to do this… perhaps the easiest is to just modify the GoObject.DragsNode property when the main node gets selected. You could override OnGotSelection/OnLostSelection on your node (see ChartNode in Classier for a sample of these overrides).
turn off Undo (SkipsUndoManager) when you're playing with DragsNode, like in this note from yesterday.
and now, for my editorial comment:
I'm not sure that having selection change the behavior of a node is good user interface design...