So, who is going to be the hero who will sacrifice by using the demo 30 times and checking if it can be downloaded again?
It won't work. I was thinking myself of various theories about how the limits were implemented... then I remembered the Activity Log.
It's obvious the demos can access the Activity Log data to check how many times they were played and such data remains even if you delete the demo or swap SD cards. The only way to clear it is to format the 3DS.