The next morning it was still quite swollen and seeping blood. I couldn't find any new trauma, and he wasn't lame, but it's definitely annoying him.
I am not a vet, but I imagine that this is what happens when you remove a pressure bandage after fourteen weeks. Rather than panic, I cold hosed the leg for ten minutes and hand walked him for fifteen. I rode him for less than ten minutes, at a walk, gave the wound another coat of Aluspray, and finished off his day with two grams of Bute.
I'll call the vet today, but I don't think there's much else to do other than re-wrap it which I'd rather not do. I think the swelling will just work itself out with light exercise, and the oozing blood will stop when there is less pressure from the swelling.
And this too shall pass ...