Maybe if there could be an element of 3-D mail without actually mailing the physical camera around?
Everyone involved in the project provides their mailing address (we might have to set up a semi-private method of doing this, although obviously everyone involved is accepting the risk), and someone takes on the responsibility of getting the ball rolling.
They do their own text, images, whatever, and post those online. Then they print it out (as much as they can given their personal printing capabilities; obviously the digital copies are online for perusal if someone lacks good-quality color printing), and mail the paper to someone on the mailing list without revealing who online.
When the person receives the letter, they get all excited as fuck and post online "HOLY SHIT YOU GUYS I GOT THE LETTER THAT MEANS IT'S MY TURN" and then they add their own stuff. Write a little blurb, a full article, draw a doodle or diagram, take a picture or three, whatever, and post it online. Then they print out whatever seems reasonable to print, and mail that to someone else on the list without revealing who online.
The idea here is to make this fun little activity where everyone who receives the letter gets all psyched and it becomes a game of Collaborative Discordian Hot Potato. Rather than trying to mobilize a dozen spags at once (which, as we all know, is a pain in the butt), only one person at a time has to do something, and everyone else gets to have fun watching while still being a part of it.
I'm thinking that this creates the dual benefits of the ease of online collaboration with the sense of fun that is inherent in receiving a piece of snail-mail that isn't a bill or credit card offer.