Courts (especially in the US) have been known to equate all kinds of things with speech, so it's not unthinkable to equate the symbolic act of NOT doing something with a form of speech. Not standing for the the national anthem or Pledge of Allegiance, for example. So in THEORY a court could rule that the act of not doing something you might be expected to do, such as include a warrant canary, is still a form of speech, and in doing so you are violating your gag order.

Still, all theoretical and purely speculation, because as far as I know this has never been tested in court.