I wouldn't presume to argue legalities, but from a practical point of view;
- calling taxiing by sat phone makes little or no difference to ATS. Instead of the HF operator calling us, the supervisor wanders over with the cordless phone. Arguably you are out of comms once you hang up, but that is no different to single VHF only aircraft once on the CTAF freq anyway.
- most if not all "no joy on HF" flights are RPT, so presumably have someone on the ground.
- if there is no one one the ground watching/waiting, surely cancelling SAR in the cct on the way in is just as bad as departing without a sarwatch?
T28D - I guess it is a case of late vs no help. If the prang is bad enough, even having ARFF and a MASH prepositioned at the the crash site isn't going to help. Opens up another debate as to how useful any sarwatch is out in the GAFA.