I honestly doubt they would come looking for fan-made costumes; it's just a matter of fact that the pros could build what they needed (on a hollywood budget) to the specs they're given, and have suits that can be reused in future movies (or bad scifi C movies.. Ala the Starship Troopers armor showing up in anything and everything over the years). Could a 405ther get lucky and end up as an extra? Sure, it's a matter of statistics and who's attempting to get jobs like that.
Take a good, hard look at the differences between H1-3 and ODST/Reach. The later games both had a strong emotional impact throughout the game that the first three struggled for. For a movie to have a broad audience, you've got to have visible faces. Think about Iron Man and how it makes a point to show Stark's face at pivotal points along the action scenes. The suit of armor itself is wicked cool, but it's emotionally cold. There's not a connection with the audience. Same with Master Chief. They would have to show his face if he was a main character in a movie, or at least enough of a face that it could show fear/anger/etc at important points.
Something with as epic a platform as Halo needs to have a trilogy. It would be cool if they started the first movie with marines and steadily introduce armor (ODST's, other human soldiers, etc; and introduce the fight against the Covenant), up through other Spartans, and finally have Chief busting in right at the end of the second movie, when everything's gone to crap and back for the characters we've been watching. If he's not the primary character - kinda like what WETA did - you get what you need for emotional attachment from the other characters, and Chief can be going full force like he does.