Earlier quoted context omitted.
The law takes the idea of forcing a person to speak much more seriously than the idea of forcing him to remain silent, particularly if you would be forcing him to make a knowingly false declaration. I can't think of an instance off-hand where a US judge has forced a civilian by injunction to make a known-false public statement.
Transparently contriving to escalate keeping a secret to making a positive declaration would, I think, not impress a judge very much. After all, it is rsync.net who is intentionally placing themselves in a position where making a false declaration is the only way to comply with the law. The judge does not need to force them to make a statement; he only needs to observe afterwards that they have communicated the fact…
There are a variety of circumstances under which the warrant canary would not be published. A warrant being served might be one (it might not), but there are certainly others: policy change being the most obvious.
I don't think there is a good legal precedent in general for maintaining that someone's failure to speak (under whatever circumstances) violates a provision for secrecy. I think that would be a hard argument to make, given that, at most, the failure to publish would lead some people to suspect that some kind of warrant had been served for something, somewhere, sometime in the past week or so, that rsync might have. Vague, at best.