The author of the Unofficial Guide to WDW/DL said that he has used a fast pass three years later. He got a weird look from the CM but was let on the ride. I don't think I would try it but it has worked in the past.
The only time they might let you use a FP the next day is if the ride broke down and wasn't expected to be back in operation that day, but you'd need to find a lead (for that attraction) to sign off on it. I did this once with Buzz.
Since they allow using FPs late on the same day as a matter of policy, it is not "unofficial". I would say a better word is "unpublicized" - at least Disney does not publicize it. But it definitely is official.
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