Gatekeeper could threaten to deanonymize the AI. Or is the gatekeeper not supposed to be actively fighting back?
The AI-player could arrange the chat session (with a willing gatekeeper) using a throw-away account. I think that would preserve anonymity from all but the most determined gatekeepers.
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.