We launched a similar sort of thing yesterday, taking advantage of our community we're sending people free stickers if they take a picture of themselves at a polling place or with their ballot papers. http://dft.ba/vote

This idea seems pretty cool, could you not just add an entry to peoples calendars (Might reduce the number of people you could influence to just smartphone users, but I would imagine most people would have smartphones. Guessing it's using Twilio?