The idea of monitoring 121.5 with a spare radio, whenever operation permits, is so that a transmission from anybody, anywhere at anytime is is heard. An aircraft at altitude over a remote area is in a unique position to relay a signal. Selcal would defeat the object and there should not be other traffic on this frequency.
If Nigel is not responding on 121.5 then he has probably turned the volume down and he, not SECAL, is defeating the object of tuning it. SELCAL would get his attention which is a good idea as a last resort if he is not responding. Most commercial aircraft are fitted with VHF and HF SELCAL, why not deploy every tool in the box to establish communication?