(javascript) --event--> (java: nope, nothings going on)

[Noel]

So, I'm designing an app for an embedded web browser (Sketchup). I
can control Sketchup by changing the window.location value to
"skp::myFunciton@myParams". That's all fine. In the other direction
things get a little more complicated. The program can execute
javascript commands but here is the catch... I want to ask Sketchup
(from my GWT application) to perform some operations and return a
result. This operation is asynchronous! The request is launched by
my entrypoint instance but JSNI can only map static functions.

I thought I had a solution with events...

//Javascript
var gwtwidget = document.getElementById("response")
gwtwidget.value = "blahblah";
gwtwidget.onchange();

and then listening for the change in GWT. Alas, it doesn't work.
Gwt's own event system overrides, sinks, deters (or whatever) the
event. What approach should I take? I've been through the web in
search of info but I certainly can't get my head round it. I'm
guessing the answer is either...

1 Call an entrypoint instance method from javascript (somehow)
2 Fire an event from javascript that will be picked up by gwt
(somehow)
3 Setup an async callback interface mechanism of sorts (somehow)