I was thinking about this recently, since a dev from one of the communities I hang around vanished without warning and we know nothing about him or how to contact him, scary stuff.
While the above is a good start, the problem of course is that this whole approach is centralized.
If you got a gag order attached to one of those NSA letters, then you'd be obliged by law to no make your compliance public (incredible nasty legal power).
Does taking down the above page count as making x public? that's a legal question.
The second problem is that it would require active change from the forum admins to add a 1 in place of the 0, where it says how many requests have come in.
One way I was thinking of doing it, is to use a time lock smart contract.
I don't know with Bitcoin, but I can imagine a not too complicated mechanism possible with NXT, and certainly with Ethereum, where you write a text post, or maybe some kind of script which would change the canary text.
Then you put that into a time lock transaction, where it will be executed in a month, unless you send a transaction to it to cancel it. Then before the 30th day, you send the tx, cancel the contract and publish a new one, this process of reissuing of the contract could be automated.
Bitcoin's multisig might be enough for this, NXT would require use of 'account approval' tech I suspect that may work. or just using Etherum.
With NXT, the message could be sent to a explicit set of addresses, so only the exact people you want to know a given piece of information after you've been silent for a month (or after you've let the canary alarm go off) would receive your message. That could include the website which might have a script to post the content of that message on a website.
The above approach would be better suited to people going MIA, but still would solve the above problem in a more complicated , cooler and yes, decentralized way.
Still that might not be immune to the legal definitions of 'contempt'.
As far as coding this thing? I'm a visionary, much more then an engineer.
