Here we are at about 4 days (96 hours or so).
It's so dark now that I can barely make out the piece of structural metal just behind where the water pump shaft would be. And that's only a few inches from my window.
So, clearly a big change in the last 2 days. Before I could see quite a ways into the head, but no longer.
I'm not in a big hurry, so I'll leave it in there a few more days to make sure all the rust is dissolved. Unfortunately there probably won't be any more photos unless enough of the iron oxide settles to the bottom and the solution clears up.