Thanks Robert, but please don’t bother. I assume the circuitry in powerbanks will depend on the brand & model, so my results will probably differ from yours anyway.
As it happens, the last couple of days I have noticed that when I plug in my powerbank it immediately starts charging - in which case the latching switch is all I need (and to personally make sure I charge the powerbank after each cycle).
The better long-term solution is (a) to invest in a solar panel which outputs more current, to reduce the likelyhood of the main Li-Po running so low; and (b) to sort out deep_sleep to reduce the time in which power is being used.
Yes we did 
So many threads of thought that link together. I will get that DFRobot latching relay in my next payday’s order - in the meantime I “just” have to research the way to program in ESPHome the two GPIO pins to drive it … but that’s over in the ESPHome forum.
Thanks for your time and help Robert.
