As of Bitbucket Server version 4.8 the core product provides a feature to add repository reviewers.
We have not fully tested the impact of this feature onto Workzone functionality, especially auto-merge conditions such as approval quota, group quota etc.
Please introduce and test the new Bitbucket Server reviewer feature first on a non-production instance and report any problems straight back to us.
Update (21/07/16): It seems to be safe to disable the Default Repository Reviewers plugin.
Thanks and happy coding
Your Izymes Team