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.