Cliff, I saw and loved the turtle two years back, so I'm happy you and your "kids" are at it again!
I, too, was worried about the prospect of some students hearing back, but others never getting anything because the postcard was lost in the shuffle that is TTITD. I think you've probably come across the best solution: collect the postcards at the station in a bin and then you, as the coordinator, haul 'em over to the BRC Post Office every afternoon or so. If you make it really clear that "COMPLETED RESPONSES GO HERE --->," I don't think you'll lose too many, unless some burner thinks the art is so cool they have to take it home for their private collection.
Another possibility would be to have the kids collaborate on bookmarks. Student A works with Student B on a bookmark, then A with C and B with D. Each student helps create at least two bookmarks, which increases their chance of hearing something back and lets them participate in that "communal effort" or whatever everyone goes on about ;P
The messages on jointly-created bookmarks may become less intimate as a result of their being vetted with a classmate, but I think it gives the students time for some really interesting discourse in groups of two or three.
Also, postcards are completely in the theme - the silk road transported information and news just as much as it transported camels and spices.
When he lights his streetlamp, it is as if he brought one more star to life, or one flower.
When he puts out his lamp, he sends the flower, or the star, to sleep.
That is a beautiful occupation.
- Le Petit Prince, Antoine de Saint-Exupéry