So, you've been working for Marvel, and you've created a character under terms not governed by the current work for hire contracts (say...oh...a throwaway character that was only supposed to be in one issue so no one thought anything of it until it got its own series and a movie). Tell Marvel's special character committee who the character is, first appearance, and so on. A special panel will review the information and determine whether or not you are entitled to additional compensation for your work.
I don't know the details, but it sounds to me like this is an attempt to circumvent arbitration and creator rights lawsuits. I'm figuring the decision to sign up is binding (read that: no going back to your lawyer if we feel your character wasn't that big a deal). So you have to trust this committee run by the company you did the work for that they will determine you deserve extra for what you did for them.