I pondered whether I should still take him, but then decided an off property trip would do him good, and a final "all clear" from the vet would relieve any lingering doubts.
Eventually, he decided to leave it alone as he didn't want to dig a big hole for nothing. After much back and forth, we decided to just leave it alone and see what happens. He agreed that riding would be a good idea as pressure on the frog increases circulation which might move the abscess out and away. Of course, if Izzy comes up lame, I can poultice it with Numotizene to help draw out the abscess. In either case, my farrier will be out in three weeks, so he'll have a chance to find what's left to be found.
And then someone else's foot was up. Check back tomorrow for that one.