Hi TomKent

One good solution I can think is to inherit XanMenuItem, add TreeItemsSource dependency property to it, and bind this property in control template of XamMenuItem. Below is a code sample how to do it.

1.       Inherit from XamMenuItem, and declare your property

    public class XamMenuItemEx : XamMenuItem

    {

        #region TreeItemsSource

        /// <summary>

        /// Identifies the <see cref="TreeItemsSource"/> dependency property.

        /// </summary>

        public static readonly DependencyProperty TreeItemsSourceProperty = DependencyProperty.Register("TreeItemsSource", typeof(IEnumerable), typeof(XamMenuItemEx), null);

        public IEnumerable TreeItemsSource

        {

           get { return (IEnumerable)this.GetValue(TreeItemsSourceProperty); }

           set { this.SetValue(TreeItemsSourceProperty, value); }

        }

        #endregion // TreeItemsSource

    }

2.       In control template bind XamTree.ItemsSource to above declared property

   <ig:XamMenu Grid.Row="1" Grid.Column="1" ExpandOnHover="False">

            <ig:XamMenuItem x:Name="_menuItemProperties" Header="Property" Width="Auto" HorizontalAlignment="Right">

                <local:XamMenuItemEx Height="Auto"  StaysOpenOnClick="True" >

                    <local:XamMenuItemEx.Template>

                        <ControlTemplate TargetType="local:XamMenuItemEx">

                            <ig:XamTree x:Name="tree" ItemsSource="{TemplateBinding TreeItemsSource}">

                                <ig:XamTree.HierarchicalItemTemplate>

                                    <ig:HierarchicalDataTemplate ItemsSource="{Binding Children}"/>

                                </ig:XamTree.HierarchicalItemTemplate>

                            </ig:XamTree>

                        </ControlTemplate>

                    </local:XamMenuItemEx.Template>

                </local:XamMenuItemEx>

            </ig:XamMenuItem>

                By this way you can control the items in tree by this TreeItemsSource property.

3.       So when you want to change the data displayed in tree you just change data set to TreeItemsSource property

 XamMenuItemEx item;

 private void ChangedData()

        {

            List<Data> data = new List<Data>();

            item.TreeItemsSource = data;

        }

If you want to get reference to the tree you can do that in OnApplyTemplate method of XamMenuItemEx class

        XamTree tree

        public override void OnApplyTemplate()

        {

            tree = GetTemplateChild("tree") as XamTree;

        }

Hope this helps

About your second point, the HierarchicalDataTemplate is part of XamWebTree (XamTree in 10.2) assembly and in this sample, it is used only for demonstration. It is not necessary to use it to populate your tree. From this link  http://help.infragistics.com/NetAdvantage/Silverlight/2010.2/CLR4.0/?page=SL_xamTree_Binding_to_Data.html you can read more about how to bind your data to tree.

Regards

Todor