There's a decent write-up here
(and a long string of comments querying the finer points of what is or isn't rebellion). Basically you just carry on like normal and nobody is really going to mind; it's not exactly the Proper Way To Do Things but if it works for you, then it works. It seems to me the most important point of the original article is
"This is a self-challenge. The REAL prize of NaNoWriMo is the accomplishment, and the big new manuscript you have at the end. Everything beyond that is icing on the cake."
As far as I can tell (quickly browsing through the article).
Novel of any sort - Not a rebel.
Connected short stories - Not a rebel.
Novel in weird format (diary, letters, etc.) - Not a rebel.
Fictionalised biography or autobiography - Not a rebel.
Unconnected short stories - Rebel.
Memoirs - Rebel.
Other non-fiction work of any sort - Rebel.
Film or theatre script - Rebel (but Nanowrimo has a separate "Script Frenzy" event held another time).
Non-written media (art, music, etc.) - Rebel.