I don't think this is possible through standard properties; since the UltraOptionSet doesn't expose any properties for centering the items, there is nothing that the grid can do.  The EditorControl is solely responsible for rendering a cell's area, though it will ask the column/cell to resolve appearances, but the TextHAlign property will not shift the actual position of the radio button.  The only way I can think of to do this would be to use a CreationFilter to manually size the option item.  If you want to go this route, the UIElementViewer might come in handy.

-Matt