After extensive physical examination, four or five different views on the rads, jogging up and back, and a consult with the farrier, the verdict from the vet is that my gut was right. Tristan just has a whopping big hole in his foot from the abscess.
It was kind of freaky to see on the rads, honestly. You don't like to see holes in your horse's foot. But we were able to clearly see that there was no involvement whatsoever with the joint capsule or the coffin bone, no hint of a keratoma, no pedal osteitis, and the big lump above his coronet band is just a particularly nasty bit of scar tissue that will need to grow down.
The farrier's opinion is that another 4-6 weeks of growth will make a big difference in his comfort level as the holes will grow down far enough to make the foot much more stable. Probably the holes won't grow out entirely for another 6 months at least. For the foreseeable future he'll need to be flushed and wrapped regularly to make sure no new debris gets caught in the tunnels in his foot and re-start the abscess.
Farrier put a shoe but no pad back on so that the foot can continue to flush properly. The vet tranq'd him for the shoeing so he would behave, and he was pretty stoned and pathetic. He got about 2/3 of the way through his soaking before he started to wake up, and since he couldn't have hay, he was pissed about the soaking. It was an adventure.
I'm glad to know that nothing truly dangerous is going on, though, even if it will be a while before we're back in action. He's losing muscling across his back, and it hurts to see. We should be back in serious work just in time to be stuck in the indoor for the winter, too.