so apparently there is a federal regulation limiting any accessories added to a helmet limiting it to 0.2" sticking out from the helmet. this would disallow any cameras and perhaps some communications equipment. if you're up for an extremely dry read, here it is which outlines the specifications and testing a helmet must be able to pass:
p.s. i have only skimmed over this, so i don't know if this is talking about what a helmet must be classified as to be labeled DOT certified, what a helmet must adhere to in states that require helmet use, or just what is required of a helmet to be legally manufactured and sold as a DOT helmet.