Hi Richard,

There's no property for this, but it could be done using a CreationFilter. This is actually a pretty simple CreationFilter, since all you really have to do are swap the X values of the element rects.

Here's a quick sample I threw together:

    public class FixedHeaderButtonOnLeft_CreationFilter : IUIElementCreationFilter
    {
        #region IUIElementCreationFilter Members

        void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
        {
            // See if the parent is a HeaderUIElement
            HeaderUIElement headerElement = parent as HeaderUIElement;
            if (headerElement == null)
                return;

            // Get the TextUIElement and the FixedHeaderIndicatorUIElement inside it.
            TextUIElement textElement = headerElement.GetDescendant(typeof(TextUIElement)) as TextUIElement;
            FixedHeaderIndicatorUIElement fixedHeaderIndicatorElement = headerElement.GetDescendant(typeof(FixedHeaderIndicatorUIElement)) as FixedHeaderIndicatorUIElement;           

            // If the FixedHeaderIndicatorUIElement is null, then do nothing.
            if (fixedHeaderIndicatorElement == null)
                return;

            // Change the X position of the rect of the FixedHeaderIndicatorUIElement so it's
            // position on the far left of the header inside the borders.
            fixedHeaderIndicatorElement.Rect = new Rectangle(
                parent.RectInsideBorders.X,
                fixedHeaderIndicatorElement.Rect.Y,
                fixedHeaderIndicatorElement.Rect.Width,
                fixedHeaderIndicatorElement.Rect.Height);

            // Check to make sure there is a TextUIElement, just in case the header text is
            // empty.
            if (textElement != null)
            {
                // Change the X of the TextUIElement's rect so that it is to the right of the
                // FixedHeaderIndicatorUIElement.
                textElement.Rect = new Rectangle(
                    fixedHeaderIndicatorElement.Rect.Right,
                    textElement.Rect.Y,
                    textElement.Rect.Width,
                    textElement.Rect.Height);
            }
        }

        bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
        {
            // Do nothing
            return false;
        }

        #endregion
    }