That's how I'm using it. I have a photography section with multiple subforums underneath the larger parenting forum I am part owner of. In the drop down menu, the main category is listed for the whole photography section (I put it's forum id in) and it lists the new posts for all of the child forums. It still not be what you are looking for, but thought I'd let you know that it does work like that.
Good luck in the hunt! We just opened up shop a week ago, so I'm learning this stuff as I go!