I've gone through some of my books but I can't find any real information.

Is there some sort of best practice involved when a fish jumps out, has been out for an unknown but extended period of time but is still alive?

One of the koi was out of the pond, but still alive. I put it back in the pond, and watched it for a bit. After around 20 minutes he seemed fine and looked to be oke. At the time I didn't worry about it. Today I came out and the koi was dead. I did a quick necropsy and saw that the gills were off a bit. Not in a parasite or disease sort of way just odd.

I don't think it was just a coincidence, but who knows maybe there was something wrong with it initially that made it jumpy..

Well, there isn't a thing you can do about physical damage the fish may have done to itself from impact. If it hurt itself bad enough, it's gonna expire regardless of what ya do. Realistically, the best ya can do is get it back in the water and do what ya can to start the recovery/healing process.

My practice is to return it to the tank (quarantine tank if ya have one available to reduce stress and prevent it from getting picked on by tankmates while in a weakened state), add an airstone to increase DO in the water (its been deprived of oxygen), then treat with something that will help rebuild the slime coat and a bacterial/fungal remedy for preventative reasons (the slime coat has been rubbed off and often the fish will sluff the rest if it's covered with dirt, lint, etc. Who knows what kinda nasties it's contacted, thus preventative mode). I used 1/2 doses of Melafix and Pimafix for the preventetive aspect on a coupla danios that hit the floor. Worked well. Personally, I don't care for Melafix or Pimafix for treatment purposes, so if the fish started showing signs of a fungal or bacterial infection, I'd use something stronger for treatment.

WYite

To Snappy's delight, I've emptied one of the 100g livestock tubs of feeders into her little pond to use for quarantine in case it happens again. I was slacking about having any kind of quarantine for all the koi here now.

It's interesting, in that common goldfish must be a lot sturdier than koi. I had separated out the goldies from the koi, and there's a few beefy ones in the one pond. One of them jumped out, and had been out so long the fins were near jerky consistency as far as being dried out. I picked it up and it was still just barely alive. I saw the gills go just a bit, so I dropped him in the tub totally certain than someone was having a feast later, but, knock on wood he's tail fin still looks like a mess, but he's acting pretty well otherwise.