Can Java objects be accessed from Javascript in GWT?

My goal is to initiate RPC calls directly from javascript. I have come up with ways to fake callbacks (because of the asynchronous nature of RPC) but I can't figure out how to get custom objects into javascript.

So, I've created a class named Interop and I statically create the service I'm interested in (had to use static as it was all I could get working, I don't think it's relevant right now):

public class Interop {
    private static final GreetingServiceAsync service = GWT.create(GreetingService.class);
    ...
}

I then create a function that will do the async calls and handle the responses:

public static void greetServer(final String success, final String failure) {
    service.greetServer(
        "Homer", 
        new AsyncCallback<String>() {
            public void onFailure(Throwable caught) {
                callback(failure, caught.toString());

            }
            public void onSuccess(String result) {
                callback(success, result);
            }
        }
    );
}

Then I create a JSNI function to export this function to javascript which I call from the onModuleLoad():

public static native void export() /*-{
    $wnd.greetServer = $entry(@package.Interop::greetServer(Ljava/lang/String;Ljava/lang/String;));
}-*/;

And also create another JSNI function to deal with the callbacks:

public static native void callback(String func, String response) /*-{
    $wnd[func](response);
}-*/;

So that the function names I pass into greetServer() initially for success and failure are called by the JSNI as callbacks. And this all works great when dealing with Strings or (I assume) a primitive type. But when I try to do this with custom types (note the altered Custom type parameter):

public static native void callback(String func, Custom response) /*-{
    $wnd[func](response);
}-*/;

Then what ends up in javascript doesn't work. It seems to be a javascript object with cascading arrays and none of the methods are available.

So, the question is, how can Java-originated objects that aren't basic or primitives be accessed from within javascript (not JSNI)? From what I can tell JavaScriptObject needs to originate in javascript, but in my case, my objects are originating in Java. What can I do?

I've also looked into gwt-exporter and that shows how to instantiate java stuff from javascript, but not how to access java-originated stuff in javascript.

I know this is a bit confusing so please let me know if you have any questions. Thanks!