We can't track a user across different domains, or customers.
To be super clear- yes you can. You don't. That's very different. With full JS access on a site you have the ability to collect a lot of information. As another poster mentioned, it only takes 30 bits of entropy to identify all 3 billion internet users.
Technically, though we can't, since we'd have to dedicate engineering time to making the changes necessary to do that kind of tracking, and we're not going to :P
