We do it a different way where we inject the uicomponent into the bean using seam, like
private transient UIDatascroller scroller;
where the UIComponent is the org.richfaces component.
Then in the code we do a....
But in your case I would have thought the value of the page property on the rich:datascroller should do the job.
Are you rerendering the "list".
I have seen the error before where you set the page but it does not get propergated back to the page, havent figured it out as yet.
you should use page binding instead of pageIndexVar which is an attribute to define request scope variable for page access to current index(like var in dataTable).