One *really* easy way to do this without needing access to the source code...
In your site's root web directory, look for showproduct.aspx.cs
Edit this file and find the method called OverrideTemplate()
Change the start of the method to look like this:
Leave the rest of the code unchanged. You will of course need your custom product template file (called product.master) in App_Templates\Skin_1
protected override string OverrideTemplate()
(Note: This switches off all other template switching for products and is a brute force quick code change)
Taking this a stage further, you could even put template names into the product's ExtensionData field. This would enable you to use the default template (where ExtensionData is empty) but to allow individual products to have their own custom templates.