I see Bob Nugent all the time, I'll ask him.
I suppose you would soak the steel wool in nitric acid to make ferric nitrate, then go through the regular process. I would consider this a cool trick if stuck in a remote locale with no other supplies, but otherwise I would just start with the ferric nitrate directly.
But I think the coolest part would be to use Iron directly off of some historic site as the source of your sensitizer to make prints representing the location. It's more conceptual than anything else.
Could we extract enough iron from human blood to make portraits? Any volunteers?