I think if you could control how many times the ad appeared to the same customer, it might be worth testing.

The annoyance and distraction factor for an ecommerce shopper trying to complete their transaction may make ecommerce sites slow to try it out.

But like you say, if it was used to drive traffic to special offers or coupon codes and not shown every time the customer got lazy with the mouse, it could work. Might even work for upselling lower priced products.