I've just released the User Session package for Meteor, which is a user-based persistent key-value store, kind of like Session. User Session allows you to store, view, delete, list, and compare key-value pairs, tied to a specific user of your website. They are stored in MongoDB, and thus are persistent across sessions and devices. They are accessible both from the client and the server.
Thanks to digilord in the Meteor IRC for his assistance, and the idea!