NaNoWrimo rules are simple:
The rules say 'novel', and we'll be writing flash fiction: won't that turn us into some sort of NaNo rebels? Maybe, or maybe not.
So, to make a long story short :), if you feel like writing 50K words of Flash Fiction in 30 days, do it!
NaNoWriMo is waiting for your prose.
Find out more about being a NaNo rebel at:
Am I a Rebel? Find out here! | National Novel Writing Month