I have tried the grid heater (used from 12v cummins). Maybe in a warm climate with a easy starting engine, but on the couple I tried it on, any temperature that would require glow pkugs normally, would not start on the heater alone.
Have also tried the ether systems, never could get the amount of ether regulated down enough for easy consistent, non locking up starts. If you got the timing of ether delivery and cranking down perfect it was pretty amazing how easily one would start. Had a few actual engine block measured temps of minus 15 that started like summertime. Pretty amazing.