I think a pretty straightforward answer is that for it to be information, an intelligent being has to perceive it as such. I don't think there's a loophole - if you would successfully send information farther back in time using any means, Time Gets Mad, and things somehow work out so that you don't. And even if you get around that, unspeakably bad things probably happen.

However, I suspect that the six hours is an artificial safeguard built into the spell. You could PROBABLY create a new time travel spell that can go seven or eight hours without breaking things. I doubt the 6 hour limit is actually the snapping point. It's just the limit to what you can do before you start to deal damage to reality. Going farther once might work, but if everyone did it then Very Bad Things happen. If the payoff matrix for "everyone defects" is that the universe stops existing, you should probably cooperate.

Edit: NVM, reread the end of the chapter, and 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. Then again, this would not be the first time Eliezer has employed the "powerful weapon being an amazingly well kept secret" thing, and it was somewhat foreshadowed when Harry mused that maybe scientists had come up with things worse than nuclear weapons.