The issue I see is what options the court has to break your canary. The general arguement for a canary is that: A) the government cannot infringe on your freedom of speach without cause (so it is legal prior to the request), and B) the government cannot force you to lie (so it is legal to remove the cannary after the request). However, with a per user cannary, once the request has been made, the court might be able to order you to remove the canary from all users to protect the legitamite government interest of maintaing secrecy about who is being targeted.