Is it OK to stop the indoor blower during heat pump defrost on split-system heat pumps?
During the defrost cycle, inside heat is not being pushed to the outside as you stated  "the parasite heat load of heat that is being pushed from inside to outside to defrost the coils themselves". The inside fan has to run to insure that the liquid refrigerant entering the indoor coil is vaporized so that liquid refrigerant does not flow back to the compressor. The outdoor coil is defrosted by the "heat of compression". The temperature of the outdoor coil is elevated by the compressor pumping the refrigerant vapor to a very high pressure which produces a very hot gas to defrost the coil. Since the outdoor fan is turned off during the defrost cycle, the refrigerant does not condense and the coil pressure rises as does the coil temperature. The colder the outside temperature the need for defrost cycles is reduced due to less water vapor (humidity) being on the air. Less humidity results in less water vapor to coat the outdoor coil. The back-up heat is turned on to temper the now cold air that is being generated by the defrost cycle.-------- Add-on to answer; actually @ThreePhaseEel my explanation does answer your question; you asked if you could add a relay to shut off the indoor fan. You can , but do you want to gamble on slugging liquid into the compressor which may screw up the compressor. The indoor fan runs because it should. The heat strips turn on to keep the discharge air at a reasonable temperature. They do not have to run for the defrost to work.