As I was writing in another thread What's your current backlog counter? - #9 by apolyton Jira has a powerful “query language” which allows you to do searches on any field. So a query for “status=done and resolution date < due date” is very easy to setup. You can beemind results of this query through Zapier.
I am not sure how this solves your problem of a varied number of tasks / week. On a week with only 10 new tasks , once you complete them you still need to plan 10 more and complete them as well to keep up on the 20/week goal.