Evidently there’s some sort of bug with MT’s subcategory code that causes it to work differently under dynamic publishing than under static publishing. In a static Category Archive, you can just create in a a list of any subcategories with the MTSubCategories tag, and it will start under the category the archive pertains to. In a dynamic Category Archive, it starts at the top level every time, pulling in all the categories and subcategories. (Got a clue that was the case on the MT boards.)
Which isn’t a problem in my production version of the new WIST blog/database, but which made testing for what I wanted to do a lot more problematic.