I created a small plugin that has a percentage chance to create code review requests on checkin. You could enhance this pretty easily to create reviews using a more complex condition (based on how long its been since they last had a review, or the size of the checkin etc) however I've found the 5% rule to be fairly successful mainly because people have gotten used to the review feature and have started requesting code reviews manually for things they feel are important.
The major gotcha I found while coding this was the need to impersonate the user performing the checkin as the creator is the only user who can close the review (which isn't very useful when its the service account).
