Hmm... Then it seems that what I had in mind is more difficult than I thought...
My idea was to enable the security lock when trying to unlock the screen (from low power screen) by pressing the power button, whereas double tapping the screen would unlock it as normal. I always unlock the phone in the second manner, while someone who doesn't know how the N9 works (most people) would try to unlock it by hitting the power button if they, say, stole the phone.
An attempt to make the phone useless to your average pickpocket