I'm not really sure what you're asking for. What do you want to do with this custom object?
What I would like to do is when on the client I call addListener and pass in the handler. I would like to pass in let's say (handler, "ID:12123") so a string.
Then on the server when I implement the addListener method
public void addListener(InvokerCallbackHandler callbackHandler)
I want to be able to extract the String "ID:12123" so I know that the callbackHandler passed into the server has a string of "ID:12123" assigned to it.