Frankly, this is the kind of thing a five-year-old would come up with. "Yo, warden, I just created a new religion that says I have to have steak and hookers every day! Respect my religious freedom!" The law isn't that brittle. It's hackable, but you have to respect judges' latitude and legal sense.
Then consider that they would come under speech restrictions through no fault of their own. It may be contrived, but they would be standing in a courtroom having committed no bad acts.
Would a judge order them to construct and publish a false statement? That's the question, but can you find case law that shows this ever having happened?
If not, then regardless of how contrived it is, it's still legally novel.
Being served a warrant would not necessarily involve fault, either, yet it would put them under speech restrictions -- restrictions against making true and public statements. So current law already accepts that people can be punished for making true and public statements without committing any other bad acts.
I don't think the judge would order them to publish a false statement. I think the judge would simply punish them for revealing the warrant and respond to their pleas of speech coercion by saying, "Hey, your fault, not mine, and you did it on purpose." I mean, they're intentionally committing themselves to either publishing a false statement or breaking the law. They've showed their willingness, indeed their intention, to do one or the other if served with a warrant. If they intended to break the law, then they can hardly argue their innocence. If they intended to publish a false statement, then they can hardly argue that the court coerced them to do so. And the fact that they already explained their cunning plan means they can't argue lack of foresight, either.
As for novelty, I wouldn't even trust a lawyer to know whether this is novel. People have been splitting legal hairs since long before Solomon. Putting the letter of the law in contradiction to the spirit of the law is hardly a novel concept; this is just one possibly novel example. It's probably discussed in the Talmud somewhere. "Ah yes, the famous story of Francois the builder, who owed his brother Juan thirty-five rubles and agreed to repay him by shearing his sheep on the fifteenth day following the next new moon, knowing full well that the fifteenth day following the next new moon was a sabbath day, on which sheep-shearing was forbidden."
There are a variety of circumstances under which the warrant canary would not be published. A warrant being served might be one (it might not), but there are certainly others: policy change being the most obvious.
I don't think there is a good legal precedent in general for maintaining that someone's failure to speak (under whatever circumstances) violates a provision for secrecy. I think that would be a hard argument to make, given that, at most, the failure to publish would lead some people to suspect that some kind of warrant had been served for something, somewhere, sometime in the past week or so, that rsync might have. Vague, at best.