{tag_rating} only works on blog post layouts. So, there's nothing built-in to BC for rating products by stars. At this stage, I'm not sure it's possible. I would normally suggest using Web Apps to create a rating for each product and embed that web app into each products' description but in order to make changes to the web app item (update the average rating, etc) it requires users to be logged in which you might not want to do.
Have you looked into third-party ratings systems like http://rating-widget.com? Seems like you might get the desired functionality with that javascript. It looks like they host everything on their site so you are dependant on their site not going down but I'm interested to see if you can use that script to get unique ratings widgets for each product.
