Here are a couple of workarounds to consider:

1. Instead of using the Product ID, use a private scheme to store the unique value your app needs.
2. If your really need to use the Product ID, you can leverage the way App Hub works. That is, the Product ID is assigned early on in the submission process. Specifically, in Step 1 when you upload your app. You can upload a preliminary version of your app and not submit it for testing. You can then use this Product ID in your code. When your application is ready for submission, you can re-upload your app, overwriting the preliminary version with the final version. The Product ID will not change. See below for sample code on how to read the Product ID at run-time.

To see the Product ID as assigned by Product ID, in your “my apps” list on the dashboard, click on the name of your submission.

Here, you can see the Product ID and the Deep Link. This will not change for this submission.

Code to read the Product ID at run time:

public static string GetProductId() 
{
    System.Xml.Linq.XElement xml = System.Xml.Linq.XElement.Load("WMAppManifest.xml");
    var appElement = (from manifest in xml.Descendants("App") select manifest).SingleOrDefault();
    if (appElement != null) { 
        return appElement.Attribute("ProductID").Value; 
    } 
    return string.Empty; 
}