Since you are not worried about radio interference, you could have a spdt switch on the magneto kill posts. when the switch is in normal, it grounds the flamethrower mag, when the switch is in "flame" it grounds the spark plug and allows the flames to fly.

you would probably want it to be a pushbutton where you can modulate it (on-off-on-off-etc) so the engine will not die.

I dont know if that harms the magnetos by keeping them grounded for so long.

Anyone?