Plenty of people do similar.
Aeration would be very important, so be sure you have a bubbler or waterfall splash. Filtering a small body of water would be important, too, as the fish pooping in there would foul it more quickly than the 5000 gal pond.
And an above ground pool would get colder faster, as well as cycle from warm to cold, much faster than the pond, so be sure to have a heater or some other way to keep the temperature consistant.
Koi are jumpers, so be SURE you cover the tank. I use PVC pipe frames covered with chicken wire. Keeps varmits out, too.