Handling commandline args in a single instance application

Question

I have a tricky question regarding the SingleInstance class on CodeProject.

I want to be able to handle commandline args after my window has loaded and before I started using SingleInstance, I handled all that in my MainWindow's Loaded event. Now, I think I'm forced to put all this in my App class, but I don't know how to access any properties or methods from my ViewModel there.

Code for the app:

public partial class App : System.Windows.Application, ISingleInstanceApp
{
    public static new App Current
    {
        get {
            return System.Windows.Application.Current as App;
        }
    }
    internal ApplicationInitializeDelegate ApplicationInitialize;
    internal delegate void ApplicationInitializeDelegate(SplashScreen splashWindow);

    private const string Unique = "Unique App Id";

    public bool SignalExternalCommandLineArgs(IList<string> args)
    {
        //This executes for each instance after the first. 
        //We can activate the window and access properties/methods of type Window, 
        //but not of type MainWindow, and, therefore, have no 
        //access to ViewModel.
        this.MainWindow.Activate();
        return true;
    }

    [STAThread]
    public static void Main(params string[] Arguments)
    {
        //Stops new windows from being created.
        if (SingleInstance<App>.InitializeAsFirstInstance(Unique))
        {
            App app = new App();

            app.InitializeComponent();
            app.Run();

            SingleInstance<App>.Cleanup();
        } else {
            //Runs each time after first instance.
        }
    }

    public App()
    {
        ApplicationInitialize = _applicationInitialize;
    }


    //I added this here while experimenting, though, obviously it doesn't do anything yet. Can we do something here, possibly?
    protected override void OnStartup(StartupEventArgs e)
    {
        if (e.Args.Length > 0)
        {
        }
    }

    private void _applicationInitialize(SplashScreen splashWindow)
    {
        //Do work and update splashWindow

        //Create the main window on the UI thread.
        Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(delegate ()
        {
            MainWindow = new MainWindow();
            MainWindow.Show();
        }));
    }
}

Edit 1

I have came up with an extremely dirty (and not so ideal) hack that gives me exactly what I want, but not in the way I think I should be doing it.

public bool SignalExternalCommandLineArgs(IList<string> args)
{
    this.MainWindow.Activate();
    this.MainWindow.Tag = args.Count > 0 ? args[1] : "";
    return true;
}

This allows me to pass the path of the file I am trying to do stuff with on to my MainWindow directly so that after everything is loaded, I can go ahead and open it. I hate the fact that I have use the Tag attribute for this as it just doesn't feel right. Any alternatives?