Just in case this is an issue, I plan on bringing a roll of orange electrical tape with me to the meetup. If anyone needs to mark their weapons, they are free to use that. However, I think that enforcement of safty-binding is rather lax at PAX. While Trooper0621 had to mark his pistol and SMG, I found that my assault rifle was not subject to such scrutiny, so I do not think that they really have a unified policy with regard to prop weapons, or at least not one as well defined as at other conventions that I have been to.
That having been said, if you go to lunch at Westlake Center, leave your weapons and helmets behind. One of the mall security there told me off (after I had eaten) saying that the helmet and prop weapon might intimidate some patrons. Other, more Halo-familiar patrons, seemed thrilled though.