You need to launch scanning through the DocumentViewer class to show the page picker. This is a bug, we will look into fixing it in a future version.  To get the page picker, launch the client using ClientManager.LaunchClient (you can use HiddenWindow=true to not display the main window), then open the doc viewer and launch scanning like this:

LaunchOptions launchoptions = new LaunchOptions();
launchoptions.RepositoryName = repository;
launchoptions.ServerName = servername;
launchoptions.UserName = username;
launchoptions.Password = password;
launchoptions.HiddenWindow = true; // Don't show the main window

// Launch the client
using (ClientInstance client = lfclient.LaunchClient(launchoptions))
{
    MainWindow mainwindow = null;

    // Find the main window.
    IEnumerable<ClientWindow> windows = client.GetAllClientWindows();
    foreach (ClientWindow window in windows)
    {
        if (window.GetWindowType() == ClientWindowType.Main)
        {
            mainwindow = (MainWindow)window;
            break;
        }
    }

    OpenOptions openOptions = new OpenOptions();
    openOptions.OpenStyle = DocumentOpenType.DocumentViewer;
    mainwindow.OpenDocumentById(documentid, openOptions);
    DocumentViewer docViewer = null;
    windows = client.GetAllClientWindows();
    foreach (ClientWindow window in windows)
    {
        if (window.GetWindowType() == ClientWindowType.DocumentViewer)
        {
            if (((DocumentViewer)window).GetDocumentId() == documentid)
            {
                // Found the new doc viewer window
                docViewer = (DocumentViewer)window;
                break;
            }
        }
    }

    // Launch Scanning
    if (docViewer != null)
        docViewer.LaunchScanningFromClient(scanoptions);
}