My chemical engineering background suggests that the pH might indeed increase slightly, and that this would happen very quickly right away, like in the first 3-5 minutes of the mash. This would be due to reaction of calcium and magnesium with phosphates to precipitate out. After that, based on the complexity of all the reactions going on, I could only guess that the pH would either stabilize completely, or might actually decrease from there while acidic compounds slowly leach more and more out of the malt husks.