a bunch of people in the Great Hall should know about the fact that one can send messages via patronus
None of them know how to do it, though. (I assume you're talking about the various students who learned the Patronus spell.)
It's easy to do for everyone who learned the Patronus spell and the Hogwarts teachers do use that way of communicating frequently. I think it should be common knowledge in the wizard world that you can send those messages.
The Defense professor highlited that sending messages via Patronus is an important part of fighting wars. Given that people in the armies cared about how to fight in groups I think it's likely that it's something that you discuss at dinner table conversations.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 88-89. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: