This is not supported scenario. We are using Html tags to render the html content over the SL application. Thus applying a ScaleTransform will not affect your html content.
With SL4 the HtmlBrush and WebBrowserControl are supporting this scenario, but only in OOB mode. We will accomodate these controls with our SL4 release, but still when the Html is rendered in Browser mode the ScaleTransform will not work.
the Telerik team