The way I see it what you are really trying is to extend the purpose of the Panel. I cannot see why you would want a unique identifier, as the object of the Panel is as unique as it guess but that's not the point here.
Since you want to extend the Panel do just that. Extend the corresponding class and give it a unique value, implement the corresponding getters and setters and then you are set. It is as simple as that
class AbsolutePanelUnq extends AbsolutePanel
{
private int uniqueId;
public getUniqueId(){
return uniqueId;
}
public setUniqueId(int uniqueId)
{
this.uniqueId = uniqueId;
}
}
Then create the object and do whatever you need.