I have had this site up for a few years, so that my friends at Uni can claim the relatively commonly known "Free regular cheese pizza with every $8 purchase". This was basically any 5 digit code that was divisible by 3.
It looks like Pizza Hut have recently changed this system so that it no longer accepts any 5 digit number (GRRR!!), and instead seems to be a 4 digit number divisble by 3, incremented by 1, then prepended by a number 2, and finally wholly divisible by 3 (e.g. 29082). Basically, only SOME 5 digit numbers that are divisble by 3 but must start with a 2. There may be more to it, but from what little reverse engineering I did do (guessing a few correct 16 digit survey codes and then trying to find a pattern in the reward codes), that seems to be it.
It also seems like some of these numbers have also been manually blocked/removed (i.e. they satisfy the criteria stated earlier, but still don't work). So if a code doesn't work, just generate another.
The site has been updated to use this new logic. Code is here, PR's are welcome. Let me know if you find a different pattern or something.