Is there a way to cause a transition through an aggregate property condition becoming true?

We've got stories modeled as collections of tasks.  Developers work on tasks, story acceptors (QA/business analyst) work on stories.  When a dev starts on a task, they use a transition to put them 'in progress' and when they're done they move them to 'finished'. 

When all the tasks of a given story are complete, the story should move over to QA.  The way I think about things, as part of this transition, the story owner should make sure that any testing notes are completed.  A complicating factor is that the story owner is not necessarily the same as the dev who finishes the last task, so I'm not sure how to model this sort of transition in mingle.

One possibility I thought of is adding a story status for 'need to add testing notes', and have a transition move the story to that status when there's no unfinished tasks.  But unless I'm missing something, there's no way to create a transition that would do something based on some aggregate property becoming true.  Is such a thing possible?

Assuming not, I think the best we can do is notify via email, or at least provide a view where story owners can see what they've got to take care of.  But I'm not sure how to build that view.  I'd want to see all stories who have a task count of 0 where those tasks are filtered by something like -is not 'finished'-.  I'd like to create a card view that has such a filter, but I don't see a way to filter by aggregate properties.

Any ideas?