Used TSP-PF ("Trisodium phosphate-Phosphate Free", yep: read that a second time) and water with a scrub brush after hosing the logs down. It seems to have worked pretty well.
Most of the suggestions were a little too rough. I wasn't trying to take the old stain off along with the ash! If I was, I'd just have corn-blasted them. And I'm not sure if michaelanthony has a weaker pressure washer than I do: at full throttle I could write my name in the logs for eternity with mine. I was tempted by the baking soda suggestion, but I chickened out.