http://en.wikipedia.org/wiki/Warrant_canary
Rsync.net does this:
http://www.rsync.net/resources/notices/canary.txt
It's not clear whether the secret courts that enforce NSLs could consider this a legitimate loophole or too cute to get away with, but I wouldn't hold my breath.
The people who do this for a living aren't stupid, and most of them probably don't even think they're being evil.
The deal with the warrant canary is the guy signs a new message every (day?), actively affirming that he hasn't been served with any such secret warrants.
What if he gets tired of it? Same with running the business? You can't be compelled to work forever, neither for free or otherwise.
tl;dr Yes, these NSL have compelled people to directly lie.
And I don't see why the courts couldn't do that, as lying isn't even a crime. Maintaining the posting isn't even actively telling a lie, you're just not changing anything from the way it was before. So I could definitely see the same court that authorized these sweeping movements also supporting prosecution if you tip your hand on these matters; it's national security on the line, after all!
And even if the courts wouldn't force you to lie, the FBI could still attempt to prosecute you for not lying, and for taking action that directly reveals your receipt of the letter. Even if the court is opposed to it, it will be a lengthy process, and because of "national security!" you may not even get that far. From the government's perspective, you're enabling terrorists through secret and underhanded messaging, so you're already behind the ball both in the judicial court and in the court of public opinion.
Take, for example, the necessarily anonymous writings from 2007 from an NSL recipient:
> Living under the gag order has been stressful and surreal. Under the threat of criminal prosecution, I must hide all aspects of my involvement in the case -- including the mere fact that I received an NSL -- from my colleagues, my family and my friends. When I meet with my attorneys I cannot tell my girlfriend where I am going or where I have been. I hide any papers related to the case in a place where she will not look. When clients and friends ask me whether I am the one challenging the constitutionality of the NSL statute, I have no choice but to look them in the eye and lie.
http://www.washingtonpost.com/wp-dyn/content/article/2007/03...
So yes, even the person who has done the most to stop NSL, has been compelled to lie directly to his friends and colleagues.
Sure you can. For one, what about people pleading guilty, when they are not, just because their case looks bad and they want to avoid a trial that can put then in jail for decades and get a plea bargain?
That's actually encouraged by the DAs.
The logistics behind this might be hard to pull off. I really don't know enough to know whether something like this is feasible, but if the concept was feasible to implement, it seems like you can just publicly announce ahead of time that the system is in place, and it's out of your hands.