If you want to react on changes in the meta model you can override the handleNotification method in the EditPart. There you can change the decoration of the figure. Let me know if you want to do this and need further assistance.
it depends on what you want to do. If you want to do this on creation only you can change the gmfgraph model and do what Hoang suggested. If you want to have dynamic behaviour you need to manipulate the figure which is not a big deal. All you need to do is to set the decoration in your code.