Froggie, can't you just go to the parking lot and put the UN letters on the cars instead?
Do takeout restaurants put flyers on people's cars in your area? Print up a fake menu on the back of the UN letter. Face the menu out and the UN letter in. That way no one will go around and remove all of them before the drivers get a chance to see them.
Problem is the web address doesn't print automatically on the bottom of the letter. You could add it though, so people will be able to check for themselves its validity. Wear casual clothes so you look like a worldly person, and a hat to avoid being recognized.
The only meanings of the word "spike" I know of means slipping some alcohol into someone's drink without them knowing, or a volleyball term. I seriously thought this thread was going to be about you going around putting whiskey into people's Shasta sodas at lunchtime. Darn.