For the current user (myself) I can access it this (non-plugin) way via the console:
var app = window.Discourse.__container__.lookup('controller:application');
console.log(app.currentUser.last_seen_at);
You can also do something like:
var u = Discourse.User.create({username: 'codinghorror'});
u.findDetails(); // obviously wait on the promise here.
console.log(u.last_seen_at);