I wonder if through APUG it would be worth trying to restart the craze for the carte de visite created in the 1850’s. The size of a carte de visite is 54.0 mm (2.125 in) × 89 mm (3.5 in) mounted on a card sized 64 mm (2.5 in) × 100 mm (4 in). Cheap to make and post.

http://en.wikipedia.org/wiki/Carte_de_visite

I’m not sure how it could work, but I’m sure there are a few people out there with suggestions.