I know of at least one major restaurant point-of-sale system whose API requires you to: 1) make requests by dropping files on the filesystem in a particular dir

I know of at least one major restaurant point-of-sale system whose API requires you to: 1) make requests by dropping files on the filesystem in a particular directory (that they poll at an interval) and 2) wait for it to deliver the response, which it does by writing to a new file in another directory. You're spot on when you say they're running about 5 years behind everyone else (if not more).