Facts
1) Harry Dresden knows how to preform a Dark Hallow
2) To summon up the spirits needed, there has to be a connection.
(The necromancers used a bunch of old relics)
3) People theories Harry will preform a Dark Hallow at some point.
4) There are a bunch of pictures of people who defended Chicago now hurried in a grave.
Thought: What if Harry uses those pictures, and the spirits they might summon up, to preform the Dark Hallow.
Just an idea...
Not sure Harry is quite pragmatic enough to do it yet, though.