Hi Ross,
It's been a long while with activity on this thread, perhaps it's been resolved. You can not use the Workproduct field which is on the Task artifact because it inherits from SchedulableArtifact . Your custom field is on the user story which is HierarchicalRequirement. So, when instantiating a SchedulableArtifact this field isn't there. I think it was already covered by Eric and Michael.
Because of that you will not be able to use an app to display Tasks with this kind of query.
However, you can use an app such as Custom Board and set it up for User Story (then this field is included and can be filtered). You can use the apps settings -> Query field to filter not only by your custom field but also by the tasks conditions that meet your need. This way you'll end up having a board of stories (not of tasks) but it will only filter the stories that have the tasks you need, in addition you can add the tasks to the displayed story.
Here is an example of a query that may come close to what you need. This query will display the stories that meet your custom field's required value which do have tasks and only these tasks which are not marked 'Ready'. Similarly , you can tweak this query to filter the tasks that have you as an owner or other similar conditions you need to include on the task:
(((<your custom user story field> contains/equal <your value>) and (Tasks.ObjectID != null)) and (Tasks.Ready = false))
I hope this helps.
Sagi