Saying "I can't answer that" is effectively disclosing that you are gagged, which is not allowed.
It's a totally insane prospect.
We had been meticulous in our care to not violate the gag in any way shape or form. We didn't want to jeopardize the case, and obviously didn't want to run into any legal trouble.
I read this as him saying he didn't want any issues raised that might distract from his lawsuit. That doesn't necessarily mean that the approach I proposed wouldn't be legal. A law that compels a person to lie is a bit more absurd than one that compels a person not to talk about something. It's pretty common to hear "I can neither confirm nor deny..." from government officials who work with secret stuff. I think chances are pretty good that would work for civilians under gag orders as well.
Assuming this even legal, I wonder if it would also be legal to have a warrant canary for each user.
Failing to post the canary message would cause people to know that you received an NSL. This is the definition of "disclosure."
If you can find an instance of such a scheme being tested in court, I'd love to see it though.
I haven't read any of the legal decisions; they're long, and I'm not a lawyer, but I see a big hole in the argument you're making. In every non-pathological case I can think of, a person receiving confidential information that they'll be prohibited from disclosing agrees to the confidentiality first. You can't send me an unsolicited letter containing information you want kept confidential and sue or jail me if I disclose that information, but the FBI can (pending the outcome of their appeal of the ruling that says they can't).
This is not a situation where someone's speech can be said definitely to cause harm to someone. Therefore the first amendment guarantees you will not be criminally prosecuted for it, though the government does not care.
Disclosing confidential information is not protected by the first amendment because it is a civil matter, not a criminal one. The federal government could (within the bounds of the constitution) bring a civil case against someone who violates the gag order, but it would be an uphill battle for them because the person violating the order did not sign a contract agreeing to keep quiet.