SUMMARY: Jake is home and fine so far. Tests negative.
Jake is home. I had to sit in the waiting room for about half an hour to pick him up and talk to the vet; busy period there. The up side was that I got a bunch of cool dog-related links that I'll post in a bit. The down side was that I could hear him barking, barking, barking back in the back. I'd have told him to be quiet. I'm thinking that the folks back there were only too glad to have him out of there. It was his "I think you forgot about me" bark, such as a couple of times when he's accidentally been shut in the garage, or wants us to open the door for him instead of using the doggie door, or wants something that he thinks is out of his reach (maybe a rawhide on a shelf that I took away from someone or set aside for later).
They did xrays of everything except his head (don't work well thru the skull) and nothing showed up. Blood work normal. Chemistry (?) test normal. Hmm, forgot to ask whether they managed to get enough urine for a test there. They did not give him any antiseizure medication (phenobarbitol etc.); my misunderstanding. He just got one dose of valium and another sedative (ACE, harking back to some email discussions I've seen about what kind of anaesthesia is safe to use on dogs...).
He seems completely normal. Stool is normal now, no more signs of diarrhea. He's still drinking a lot, as he was yesterday (which I also attributed to all those goodies he ate).
Vet says that I can talk to my regular vet about brain scans or spinal fluid analysis. Also said that a first seizure at this age isn't good--usually indicates a problem with the brain (but could also be heart or something else causing lack of oxygen to the brain). If he has more than one seizure in 24 hours, take him in to the hospital. If it's less often, as long as they're not very long, then I can decide to just deal with it or talk to the vet about antiseizure drugs (a la epilepsy).
We shall see what time will tell. I think it's an uneasy coincidence that him getting into everything in the front of the car was followed by the seizure--but it's also awfully close in time to that very brief staggering episode 10 days ago.