Hmm, so let's play with Diffie-Hellplant. So you have two inbred genomes as two secrets, persisted as individual organisms. Alice and Bob negotiate on a public inbred genome that they pull from some kind of seed store (being careful to never re-use one they've used before!). Alice and Bob both create their own crossbreeds deterministically, from the secret organism and the public one. It's hard to get the private genomes from the two crosses, so they can send the seeds in the mail to each other.

After that, I'm not sure you can get two shared secret clones from the crossbreeds and the opposite private inbreeds. (ETA: Pretty positive you can't, though if anyone has any ideas along these lines, I'm eager to hear!)

Sounds like the start of some cypher-biopunk novel, though.

Relevant to the article, too. It's set in a post-carbon dystopian Bangkok, where giant agribusiness monopolies dominate the world through control of the food supply, including offensive bioengineering to create mutant plagues that destroy native crops and introduce dependency. The main character is an agent of one of these agricorps, whose mission is espionage against Thailand's central seedbank.