Harry tells Draco:
Harry closed the book and put it into his pouch. "Chaos and Sunshine both have soldiers that can cast corporeal Patronus Charms. Corporeal Patronuses can be used to convey messages. If you can't learn the spell, Dragon Army will be at a severe military disadvantage -"
That happens in chapter 47. I doubt such an event would happen without Draco asking other people whether the Patronuses can be used to send messages and making sure that people in his army know about it.
I don't think Draco would be willing to reveal that he can cast the Patronus. Not after he publicly didn't even try to learn it, and presumably told everyone else what he told Harry, that it was a "Gryffindor spell". Harry had to cash in a favor just to convince Draco to learn the Patronus at all, and we haven't heard of him using it afterwards.
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: