Passing params to and from /user/login with SaltedHashLoginGenerator?

I've used SaltedHashLoginGenerator to protect a few actions within a
controller and all works well.

The only issue is, I need to pass a param that is known before being
redirected to /user/login to the /user/login page and then passed back
to the redirection page after successful login.

For example,

A user visits http://whatever.com/home/index?foo=bar

User then clicks a link (/home/content?foo=bar) that is protected with
the LoginGenerator.

When being redirected to /user/login, it should pass @params['foo']
and add it to the form submission as a hidden field.

When a user clicks login,

The page they are redirected to should also konw about @params['foo'].

Any help would be greately appreciated.