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.
EDIT: One situation where the government cannot compel you to lie is if it violates your fifth amendment rights (self incrimination).
At the end of the day if uncle Sam demandeth, uncle Sam will haveth.
I think that would be outside what can be done with a subpoena. It would require a court order.
If a mom-and-pop shop or open source org, it's a faint hope at best.
https://www.theverge.com/2014/9/18/6409575/apple-warrant-can...
The problem with a warrant canary is there's too much doubt about why it disappeared. Did they actually receive a warrant, or is it just a decision from corporate to discontinue the practice?
The result is the same.