While it was vaguely hinted at during an interview at Halo Fest, I believe the chance for any additional armor (including the GRD helmet) arrived with the TU. There’s a good chance that if there is to be additional armor pieces with H:CEA, 343i included new armor compatability with every map in the TU, incognito of course.
If not, there is a solid chance it has been programmed into only the incoming 7 maps.