Apparently the vet said something like "oh, wow" when confronted with Tristan's foot today.
The infection is pretty bad. He's off at the trot. The protocol going forward is as follows:
1) Keep soaking, epsom salt + betadine + warm water. There may indeed still be a walled-off abscess above the coronet band that will need to burst.
2) Keep the hoof wrapped at all times. No matter what. Nothing can get in there.
3) Along with that, make sure the hole is thoroughly flushed and cleaned out whenever the wrap is off.
4) The vet is mailing me an antibiotic called metronidazole. This acts specifically on anaerobic bacteria, like he's got filling his hoof. When it arrives, I need to mix water with the powder and create a paste the consistency of toothpaste, then pack that in the hole(s), then cover with gauze, then wrap with vetrap/elastikon, then duct tape.
5) Probably he will be on stall rest, or at the least very limited turnout. This is not a function of injury per se, rather that I am not confident his foot will stay wrapped if he gets too active in turnout.
I'm trying not to be too worried. We'll proceed with this protocol, and then check back in with the vet next week. If the infection doesn't show improvement, we may have to talk about cutting away some of the hoof wall to expose the anaerobic bacteria. If that happens, we're talking about months of recovery time. :(