Better yet, make each canary in the form of an original haiku.
Month 1:
reddit.com
has not to date received
an NS letter
Month 2:
This, our company
As of the time this appears
Has not been silenced
Month 3:
Our routine remains
A haiku for you from us
On the month's first day
Is the USG going to claim that reddit must, in perpetuity, write a new haiku each month? That seems like a burden designed to be defeated in court.