> It's pretty well established that they're legally allowed to prevent me from conveying a certain state -- ie, that I have received an NSL.
By compelling you to lie? When and where was that "pretty well established"?
By compelling you to avoid saying something. If you have a prearranged signal, you don't get to say "I'm just keeping my silence" any more: You are triggering a signal.
As far as I'm aware, this hasn't been tested in court. However, given that courts have been pretty silent on the idea of secret laws and compulsory silences, if it did get there, I'd be shocked that courts didn't rule this way.
If I already started a warrant canary, the only way you can compel that to not work is to compel me to continue posting. But you can only do that after right now, so you cannot possibly redact the canary itself. And you could only compel me to keep posting if you'd already served a warrant.
It's pretty well established that they're legally allowed to prevent me from telling people that I am in a certain state. It's not a stretch that prearranged signals fall under that.