How about adding to the Control Panel, Order record, a button adjacent to the purchaser's email address if the "Review Module" is installed. When selected, this button "Request Review" would send an email to the person that placed the order asking them to write a review regarding the product(s) they had ordered.
Perhaps a new text box field would be added to the Settings Field for Reviews that would have some default text added automatically in the generation of the email. Maybe even a second text box field that would also contain a default "Subject" for the email.
When the user selected the button, it would pop open a window with the recipient's email, default subject and default message body text. Sender would have ability to customize text before hitting "Send" button to send out email.