I don't know if this will help. But can you ask for an email listing your new duties. That way if its outside your duties you were hired for that you can negotiate a raise with HR.
Or at least a title change of your choosing? This way you can apply for a new job under your new title.
I'm afraid it wouldn't help... I'm state's salary, so I have a fixed salary with strict rules (qualification + professional experience for the base salary + estricted possibilities of plus payments).
Even if they gave all they could, my salary would be nothing in comparison with my boss' one. But I'm thinking about talking to some responsible person about it.