With XC Classic I could see this getting messy.
How would you create a URL for a product that is in two categories? Would it be duplicate content? Or would only the main category be entered into the URL? If only main category, what happens when a customer finds the product through it's Extra Category and not the Main Category, would it look bad to customers/google?
Just curious, because if this turns out to be very helpful for SEO, we would be interested in purchasing the code as a module if its completed
[Hint Hint to developers!]