Ahh I see what you mean... and that is the way that TaskPaper 2 queries worked. But I don't think it fits into the TaskPaper 3 query model very well. In TaskPaper 3 when you want to start talking about the item hierarchy you need to use some sort of path operator to describe what part of the outline you want to search.
The TaskPaper 3 way to perform that search is actually shorter, but maybe less user friendly:
Does that get the results you wanted?
TaskPaper 2 items all had a "project" attribute so that hierarchical queries could be made even though the query language and evaluator didn't understand hierarchies. In the TaskPaper 3 model the query language does understand queries and so items no longer have a build in "project" attribute.