With an add-in, you could do this by adding a CommandBarButton to the
"Item" menu, and then show/hide the CommandBarButton in a
SelectionEvents.OnChange handler.

I did this a while back to display a menu item for files that had a .aspx
extension. Below is the a code snippet from the main add-in class derived
from IDTExtensibility2. This should give you a good place to start. Note
the btnDavid and selectionEvents variables are members of the Connect
class, to ensure they don't get GC'd.

Ed Dore [MSFT]

This post is 'AS IS' with no warranties, and confers no rights.

    public void OnConnection(object application, Extensibility.ext_ConnectMode
connectMode, object addInInst, ref System.Array custom)
    {
        applicationObject = (_DTE)application;
        addInInstance = (AddIn)addInInst;

        // Add a "David Test" menu item to the Item context menu
        if (btnDavid == null)
        {
            try
            {
                Object objMissing = System.Reflection.Missing.Value;
                CommandBar cmdBarItemPopup = applicationObject.CommandBars["Item"];
                btnDavid = (CommandBarButton)
cmdBarItemPopup.Controls.Add(MsoControlType.msoControlButton, objMissing,
                                                                           
 objMissing, objMissing, objMissing);
                btnDavid.Caption = "David Test";
                btnDavid.FaceId = 1845;
                btnDavid.Click += new
_CommandBarButtonEvents_ClickEventHandler(btnDavid_Click);
            }
            catch (Exception e)
            {
                Debug.WriteLine(e.Message);
            }
        }

        // Add a selection events handler, to only display the "David Test" menu
item
        // when a single .ASPX is selected in Solution Explorer.
        selectionEvents = applicationObject.Events.SelectionEvents;
        selectionEvents.OnChange += new
_dispSelectionEvents_OnChangeEventHandler(this.OnSelChange);
    }

    public void OnDisconnection(Extensibility.ext_DisconnectMode
disconnectMode, ref System.Array custom)
    {
        if(selectionEvents != null)
            selectionEvents.OnChange -= new
_dispSelectionEvents_OnChangeEventHandler(this.OnSelChange);
       
        if (btnDavid != null)
        {
            btnDavid.Click -= new
_CommandBarButtonEvents_ClickEventHandler(btnDavid_Click);
            btnDavid.Delete(null);
            btnDavid = null;
        }
    }

    // only make the menu item visible when one and only one .aspx file is
selected
    // in solution explorer pane.
    public void OnSelChange()
    {
        btnDavid.Visible = false;
        if (applicationObject.SelectedItems.MultiSelect != true)
        {
            SelectedItem selItem = applicationObject.SelectedItems.Item(1);
            ProjectItem projItem = selItem.ProjectItem;
            if (projItem!=null)
            {
                if (projItem.Name.EndsWith(".aspx") || projItem.Name.EndsWith(".ASPX"))
                    btnDavid.Visible = true;
            }
        }
    }

    private void btnDavid_Click(CommandBarButton ctrlBtn, ref bool Cancel)
    {
        Debug.WriteLine("David menu item executed");
               }

<end of code>