The down side of having the glow plug powered all the time is that it changes the timing at higher rpm, which in some cases can cause problems like overheating, detonation, depending on your setup.

Using the same formula of battery-switch you could always use a microswitch that is activated by the throttle arm at lower rpm where the glow is needed.
You find these sort of switches in old dvd drives.
Although I don't use this setup for glow drivers I do use it for lighting switches.