The blood work was normal, except that two liver enzymes were slightly elevated. The vet's best hypothesis at this point is that Pie is having bile duct pain - these two enzymes are indicative of that. The pain could be due to a small bile duct stone that is irritating/inflaming the bile duct exit, but not blocking it - if it were blocked he would be much more ill, even jaundiced - or an infection due to gunk accumulating in the bile duct, or both. The intermittent acute pain and the timing just after feeding time would also be consistent with that, as the irritation would be greater at that time. Here's a description of what may be happening. Horses do produce bile continuously, so that could also explain his general low-grade grumpiness.
So the treatment plan is Uniprim daily (apparently some horses are on antibiotics for up to 6 to 8 weeks to clear this sort of thing), and a 500-lb. dose of Banamine twice a day for 5 days and once a day after that. We'll continue the Gastrogard, but not add probiotics at this point as they can result in firmer manure which is the opposite of what we want at this point. No riding at least until blood work is rechecked in about 10 days, but he can have his chiro appointment, which will probably feel good. And I'll be keeping a very close eye on him, and if there are any more acute episodes the plan may change, but for now we've got a plan - let's hope it works.