My understanding is that the sat itself is NOT measuring any BFO or frequency offset.
It is done by the aircraft itself, that reports its frequency correction to the ground station as parameter in the downlink packet.
The frequency correction can be either calculated or measured from P channel.
It is certainly known to the investigator which method the specific 777 sat unit onboard uses.
The 777 "test" flights that have taken place were not to calibrate a specific frequency offset vs. a geo position/ track / speed. This would have required to fly a 777 exactly the same route that is estimated to be flown by MH370. The flights were only to confirm the general logic and pattern.