Is there any precedent for people not facing legal consequences for failing to update the canary? The subpoena probably says "and also update your warrant canary to say there were no legal requests." Now you're in contempt of court and in jail for 5 years while you wait for your "compelled speech" case to go to the Supreme Court.
In general, I think it usually goes poorly when programmers invent clever legal workarounds. The legal system isn't a computer program. It's guys with guns.