Yes, the work needed is on Tarsnap's side, not the Stripe-interfacing bits. I need to attach Stripe IDs to accounts, find out when and how much people want their accounts to be automatically recharged (and store those parameters), write code which checks each account to see if it needs to be recharged, etc.

This is welcome news. I like tarsnap but really don't like the possibility of losing all my tarsnap backups because I am not around to recharge it within the small window. I might be out on an mountaineering trip and be away from the email alert or something else like that.