It appears you can just change the snippet to say "rating=5 star" but is there a way to collect reviews and aggregate them into the rich snippet?
Yes, for example Signs.com is a commercial printing company that aggregates the reviews across their site into an aggregate review rating that shows up on every page of their site. A bit of a gray area since in the search results they make it seem like every product has thousands of reviews, when in fact that's the sitewide total.
The way to collect and aggregate reviews dynamically would need to be created by your web developer. Showing these reviews via rich snippets would require you or your SEO to work with the developer to get the proper Schema.org formatting around these aggregate reviews.
Hope that helps, let me know if you have more questions!