JQL query to return certain issues and (only) THEIR children. (Make the argument of JQL operator "IN" able to be an evaluated expression.)

I really need the argument of JQL operator "IN" to be an evaluated expression, if I'm to create useful dashboards of work progress in complex issues (such as new product aunch blockers). (I've not found any other way to meet this request, and it limits the usefulness of sub-tasks, which is the only real issue hierarchy available in JIRA.)

For example, PARENT IN (KEY1,KEY3,KEY8) is a useful expression to find sub-tasks related to a known list of top-level issues, but PARENT IN (TYPE = PROBLEM) would be much more powerful, as I wouldn't need to know in advance which issueKeys are of type "Problem". Also, if this syntax existed for finding subtasks, it could then be leveraged to provide the sort order requested in JRA-9197.

Thanks for a great product!