Post notices are a good way for moderators to help encourage better content. However, since these notices do not directly alter the state of a question or answer, can we allow experienced users to add post notices? This can further encourage high-quality content, and is a feature that is not likely to be abused.
I personally see this as a 20k trusted user ability, but feel free to discuss the reputation threshold. In the unlikely event abuse occurs, a moderator can delete the notice in response to a flag(s), and we can control this by implementing a rate limit of, say, 2 hours (with a 5- or 10-minute grace period) for adding or removing post notices.