Capturing custom URIs in Windows 8.1 RT's WebView

Question

I am porting an application from Windows 8 Phone to Windows 8.1 RT, and I have a web service that returns a custom URI that cannot be processed by Windows RT's WebView, but can be handled by Windows 8 Phone's WebBrowser.

It is handled in the Windows 8 Phone app this way:

private string _successUri = "app://transactions/payment/complete";

void webBrowser_Navigating(object sender, NavigatingEventArgs e)
{
    var navigatedUri = e.Uri == null ? "" : e.Uri.ToString().ToLower();
    if (navigatedUri.StartsWith(_successUri))
    {
        e.Cancel = true;
        App.NavigationService.Navigate(new Uri("/Views/TransactionSuccessPage.xaml?transactionid=" + _transactionId,             UriKind.RelativeOrAbsolute));
    }
}

From what I understand, Windows 8 Phone's WebBrowser control will allow attempts to navigate to unsupported URIs, considering the implementation given above.

I have attempted to use Windows RT's WebView.NavigationFailed, but to no avail, as the WebViewNavigationFailedEventHandler only returns the web service's URI (POST, if I'm not mistaken), and not the unsupported URI.

My research has pointed me towards WebView.UnsupportedUriSchemeIdentified, but unfortunately it is only available for Windows 10.

Is there any other way to handle this?
Do note that I do not have access to the WebService, so any manipulation on that end is out of my reach.