Indeed. Military applications that require the type of precision in orientation measurements and derivatives that are being discussed here use high-rate precision Inertial Navigation Systems. INS measurements are made in a coordinate system fixed to the platform. All geometry calculations happen in this coordinate system. GPS and other sensors are used to translate positions in the platform coordinate system back to a "real world" system like lat-lon or ECEF.
This is actually a problem I have been thinking about a fair bit for the Rift. I'm not sure what the best way is to solve it in this package size. I'm thinking a first pass will need some sort of off-board reference.