Blind people, then.
Most text terminals can emit sound. You can do a lot with just beeps. The point is that it could probably find a communication channel we don't know we have.
there don't seem to be any universal-to-all-humans hacks out there.
I'm pretty sure there are many. Consider that a complex system not explicitly designed to be secure against a specific threat in its informational or physical space is almost always vulnerable to it, and human brain did not evolve to resist an attack by a superior mind.
if you have built an AI that can do all that
You haven't. It evolved in unpredictable ways after you let it run inside the box.
I meant literally universal to ALL humans. Like, ones in comas, ones who are blind, deaf, autistic, mentally handicapped, super-savants, trained in rationality, illiterate... there's such a huge variation in hardware that I can't see there being a single guaranteed exploit.
Sure, it's possible that each individual has exploits, but that's very different from a universal exploit, or even simply all being exploitable over the same channel.
In this case, it was "arbitrary text string". If the AI can make sounds, that's a new channel. It's really not h...
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.