So why not publish positive canaries with an explicit expiry date that requires manual republishing? If you set in place a scheme, long before any agency contacts you, whereby you manually publish a statement saying you have not received an NSL, and that statement disappears on its own, how could they prosecute you for failing to publish that statement?
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.