alternative to updating UI from AsyncCallback

Question

Using ASP.NET...

In my code-behind, I make an async call (BeginMyMethod) to a web service.
In the AsyncCallback method, I call EndMyMethod to get the results.

I understand the page by now has totally rendered, so I can't update the UI.

Is it possible to update the UI using these Begin/End methods?
Is there a different route I could take that would give me async capabilities as well as access to the UI?

[What I would really like to do is a Response.Redirect with the resulting value in my AsyncCallback.]

Thanks.