No, I believe you can only specify the pisition relative to the control it is extending.
Come to think of it, the
Modal Popup is probably much better suited to what you want to do (
see video here). It is totally stylable, you can put anything you want in it, you can position it in the center of the screen, and best of all, it works "modally" just like the JavaScript confirm dialog -- in other words, it forces you to make a choice before returning control to the UI.