You can get a list of the elements on the editor if you access your parent
edit part.
The XXXEditPart that contains all the elements in your diagram (accessable
also over the active editor in eclipse or the parent of your editPart that
is on the diagram). This EditPart has a getChildren Method that returns
you all EditParts.
However, when you have editParts that are hidden, you need to access the
NotationView to get all children, including hidden children
(XXXEditPart.getNotationView().getChildren()).