I am sorry, but there seems to have been a misunderstanding. It is not possible to define a function that is automatically executed after loading in yEd.
The idea I mentioned in your previous post is for a manual approach. After loading your diagram, you can e.g. use yEd's structure view to display the values of your custom property and then select the entries in the structure view (which will also select the corresponding nodes in the editor) or you can use yEd's properties mapper to highlight/transform the elements in question.