We know Hermione didn't send herself a signal she could notice instantly on a second iteration. As of ch76, we do not know if she sent herself a signal not instantly noticable.
You might not want to use a signal that could be detected by yourself during an obliviation event in order to make sure the signal isn't telegraphed to the obliviator.
Harry might think that if one needed to signal obliviation, it might be best to detect it safely in the future, unless he thought he could make use of an instantly detectable signal and a tactical response would be worth risking interception. In Ch. 6 he risked interception of the signal (he told the potential obliviator McGonnagal about it) in order to forestall obliviation. I doubt that that tactic would work with Mr. Hat and Cloak/Ms. Veil/...
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
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: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
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: