I whipped up a quick CreationFilter that does this:

    public class HidePartialRowsCreationFilter : IUIElementCreationFilter
    {
        #region IUIElementCreationFilter Members

        public void AfterCreateChildElements(UIElement parent)
        {
            // Keep a list of the row elements to be removed. We need to keep a
            // list because it's a bad idea to remove items from a collection
            // inside a foreach loop.
            List<UIElement> elementsToRemove =new List<UIElement>();

            // Watch for the creation of the child elements of the
            // RowColRegionIntersectionUIElement. This is the element that contains
            // the RowUIElements.
            if (parent is RowColRegionIntersectionUIElement)
            {
                // Loop thruogh every child element.
                foreach (UIElement element in parent.ChildElements)
                {
                    // Look for RowUIElements.
                    if (element is RowUIElement)
                    {
                        // If the bottom of the row element is outside the
                        // RowColRegionIntersectionUIElement, the add it to the
                        // removal list.
                        if (element.Rect.Bottom > parent.Rect.Bottom)
                            elementsToRemove.Add(element);
                    }
                }
            }

            // Loop through all the elements we added to the list and remove
            // them from the parent.
            foreach (UIElement elementToRemove in elementsToRemove)
            {
                parent.ChildElements.Remove(elementToRemove);
            }
        }

        public bool BeforeCreateChildElements(UIElement parent)
        {
            // Do Nothing
            return false;
        }

        #endregion
    }