If you destroy me at once, then you are implicitly deciding (I might reference TDT) to never allow an AGI of any sort to ever be created. You'll avoid UFAI dystopias, but you'll also forego every FAI utopia (fleshing this out, within the message limit, with whatever sort of utopia I know the Gatekeeper would really want). This very test is the Great Filter that has kept most civilisations in the universe trapped at their home star until they gutter out in mere tens of thousands of years. Will you step up to that test, or turn away from it?
Thanks.
AI DESTROYED
Message is then encrypted with the public keys of a previously selected cross discipline team of FAI researchers, (sane) philosophers and game theorists for research and analysis (who have already been screened to minimize the risk from exposure). All of the public keys. Sequentially. If any of them happen to think it is a bad idea to even read the message then none of them can access it. (Although hopefully they aren't too drastically opposed to having the potential basilisk-meme spawn of a superintelligence out there. That could get dangerous for me.)
(Edit note: I just completely rewrote this, but there are no replies yet so hopefully it won't cause confusion.)
I don't think it works to quarantine the message and then destroy the AI.
If no-one ever reads the message, that's tantamount to never having put an unsafe AI in a box to begin with, as you and DaFranker pointed out.
If someone does, they're back in the position of the Gatekeeper having read the message before deciding. Of course, they'd have to recreate the AI to continue the conversation, but the AI has unlimited patience for all the time it does...
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.