Using PassportJS, how does one pass additional form fields to the local authentication strategy?

There's a passReqToCallback option that you can enable, like so:

passport.use(new LocalStrategy(
  {usernameField: 'email', passReqToCallback: true},
  function(req, email, password, done) {
    // now you can check req.body.foo
  }
));

When, set req becomes the first argument to the verify callback, and you can inspect it as you wish.