It looks like I could just download newer .bin files from NGS and create my own .gdc file to reference them, to gain access to the 2009, 2012, or 2012A geoid, but I'm concerned that they were ommitted for a subtle reason that means that will not work.
I intend to use the CsmapReprojector to do ellipsoid/geoid conversions using them. Is there something I'm missing, or can I just make the new gdcs as I suspect?