Thanks - the problem is that the remainder of the page is also likely to be somewhat duplicate - it's mostly NAP data, business tagline or coupon offers, which are likely to be extremely repetitive as these businesses often have dozens of listings, all with the same info. The reviews are really our only differentiating content from all their other listings.
If the reviews popped up on their sites as well (via our widget) would a link back to their profile page (part of the widget) be enough to indicate to Google that we were the original source? I don't think canonical tags will be an option.