it seems like if there's any way to go more than six hours, period, it's such a well kept secret that no one thinks its possible

Yet Dumbledore says that someone erased Atlantis from time, and I'm guessing Atlantis existed for more than six hours. (Also, it was erased from time in such a way that people still know about it).

We know that earlier wizards were stronger, yet the book says "time turners can't go back more than six hours" multiple times, so it seems an important fact and widely believed by the characters. We know that strong Dumbledore can't do it, nor can efficient Quirrellmort (or he wouldn't suggest to Harry the plot with Bulstrode), and that time travel can be blocked long-term by the Wizards who created Azkaban. I think it will be a matter of partial-transfiguration style understanding for Harry to find some kind of loophole.