Modifying the scroll direction of RadRotator cannot be achieved with client-side script, because changing it requires many calculations and settings to be applied, which can only be done on the server.
Nevertheless you may dynamically change it through AJAX. In the attached sample page you can examine implementation of your scenario, using ASP Button to set new scroll direction server-side and RadAjaxManager
to AJAX-enable the change.
On a side note - I would recommend updating RadControls for ASP.NET AJAX to benefit from the improvements in the newer versions as well as to take advantage of the new features.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!