It looks like the previous post link was for the wrong issue - this is the correct one, which shown status is Fixed for Civi 4.7.31: https://issues.civicrm.org/jira/browse/CRM-21485?jql=text%20~%20%22renewal%20taxes%22
I got it working - follow the links on the issue, click files to see the required code to be added. Sales Tax was added on a backend renewal after applying the code.
