Expandable Overlay

Is it possible to create a layer in a widget that expands over the top of the page (beyond the width of the base widget). I've seen this done with banner ads, but am not sure if the widget wrapper will restrict the size of a larger transparent overlay. So for example, if the widget is 200 px wide in the right sidebar of a blog, and a user clicks to expand, an 600px overlay would popup over the rest of the page.