Code

diagramView.PreviewMouseMove += OnViewMouseMove;

void OnViewMouseMove(object sender, MouseEventArgs e)
{
    var viewPoint = e.GetPosition(diagramView);
    var diagramPoint = diagramView.ViewToDiagram(viewPoint);
    var link = diagram.GetLinkAt(diagramPoint, 50);
    if (link != null && targetLink != link)
    {
        targetLink = link;
        if (itemUI != null)
            grid.Children.Remove(itemUI);

        var uiPoint = link.EndPoint;
        uiPoint = diagramView.DiagramToView(uiPoint);
        uiPoint = diagramView.TransformToAncestor(grid).Transform(uiPoint);

        itemUI = new Button { Content = "test" };
        itemUI.Click += (ss, ee) => link.Stroke = Brushes.Red;
        itemUI.Margin = new Thickness(uiPoint.X, uiPoint.Y, 0, 0);
        itemUI.HorizontalAlignment = HorizontalAlignment.Left;
        itemUI.VerticalAlignment = VerticalAlignment.Top;

        grid.Children.Add(itemUI);
    }
}

Button itemUI;
DiagramLink targetLink;