I'm thinking about a plugin which get all the creating instance, then save the uid - instance reference. So that I can use expression: picker.GetX(uid) to get the x position of instance.
Because that engine can add a destroy callback in plugin, plugin can remove the instance reference to prevent memory leakage.
But,... engine has no way to add a create callback. So maybe a behavior is better then a plugin. Behavior can have onCreate and onDestroy.