If they don't have any forum rules posted that prohibit fanfiction, go ahead and post it. I don't think that anyone would blacklist you for posting a fic unless they told you beforehand not to; unless they think you can read minds. Worst they would do is delete it. Do you post in the fanfiction section on halo.bungie.org? C.T. Clown

Private Message them through Bungie. That's your best bet.

How do i ask them?