We CAN do this. We just need to resolve a handful of things before something this awesome can happen.
The biggest issue is Bungie and Microsoft. I assume, however, that the 405th can not become legally sanctioned w/o their permission, and that it will be resolved once we go official. Considering this is all copy-right infrignment, we can't pull it off w/o their support. I know the 501st got permission from George Lucas (he's a member!), and we probably would need that as well.
The next big issue is that we don't have that many people to effectivly pull off. We are mostly hanging around conventions and such, and a parade at the current stage is unlikely. However, once we recieve legal recognization we can advertise on Google, EBay, Bungie, HBO, etc... so we will likely attract a larger number of people.
The third big issue is our uniformity, or lack therof. We just don't have a single uniform everyone has to build. What I think the best course of action would be is when a new member joins, he/she recieves a file containing MJOLNIR Mk6/Mk5/Marine pepakura and very detailed instructions on how to build and paint it. Normal members won't be required to create the armor, but anymember wishing to go to a parade must have a certified suit of armor.
An easy thing to do for a parade would be to give each new member plans for marines armor, seeing as they are the simplest and most likely to be on parade. A group of people with really good marine armor could be requested to upgrade to ODST armor, adding more variety. And maybe a group of 2-4 SPARTANs could be present in each parade as well.
A fee would (I saw Adam mention that it may be something like $50) also allow the 405th to purchase a ROBOcraft printer and, as members join and pay, send each precut armor files, t-shirts, patches, etc.
The 405th has HUGE potential, and we just need to harness it.