Well, there's only one way to find out if there's a solution -- contact cperciva and ask him :)
EDIT: Wow, got a response in less than 5 minutes:
It's not something I'm looking at doing right now. The way the Tarsnap server side is designed, in order to keep costs low (and performance high), data is aggregated between multiple Tarsnap users and stored in S3 as large chunks; keeping each user's data segregated would add a lot of additional complexity and cost.