I'm not quite sure, if I understand your question correctly.
Are you looking for Requests in Status "reopen" which do have all Tasks complete?
This would not be possible, because we don't have a query syntax available, which is able to do a comparison to a count of related records( check if all related Tasks are inactive/complete).
Nevertheless, sometimes you could use query parts like an "if exists" condition.
If my above assumption of what you are looking for is correct, you might try the following scoreboard query:
type=\'R\' and active =1 and status.sym=\'reopen\' and persistent_id.[cr]workflow.status.task_complete=0
This query will return all active requests in status=reopen , which do have at least one related task within a status, which does not flag the task as complete.
This condition is more or less a synonym to your original question,as far as I understood.
Hope that helps
............Michael