Until the 405th really gets organized and has a charter and all that legal stuff, you should be fine to wear your armor at various CHARITY events. Marching in a parade is usually okay, but being in a parade on a business-sponsored float may not be, because that could be looked at as advertising. Bungie may not mind being associated with the MS Walk, but they probably don't want you in the MS Walk as part of the "Crazy Al's Pawn Shop and Loan" crew. We all want to wear our costumes every chance we get, but we should use some discretion when picking events. Erring on the side of caution is better than getting a C&D letter from Bungie. If there's any sort of business-sponsorship invovled, it's best to stay away.
Bungie made Halo to make money. Master Chief and all parts of Halo belong to Bungie (and Microsoft), so they have the ultimate decision on how their property is used. If they feel that a person or organization is abusing or 'damaging' their property, they can follow up with legal action. That means if at anytime they feel the 405th isn't using their property in a proper manner, they can sic the dogs on us and shut us down as an organization. But in all honesty, they can't keep you from making your own armor, and they can't (really) keep you from doing charity work in your armor. But, like I mentioned, they can keep the 405th from doing stuff.