You guys:
I can see you're thinking along the work of S1r9.
But AlaskaStar suggests something even easier than what I posted
in my projects folder about using a second transformer to inject
current, or s1r9's use of an inverter, which is still powered from
your battery/alternator system. In other words all the power you
need is already there.
Imagine a transistor and diode shorting from the battery to the HV coil
output, across the electronic ignition. You only want current flowing this
auxillary route when the coil is NOT firing, in other words low V,
high amps between the HV spikes. (The diode protects the transistor
from HV backflow.) Maybe a PNP transistor will let you do this, is a simple
logic function. The idea is to have strong amperage and HV to the
plug. They will be out of phase but may not matter at the business end. A
cap could correct this but you might never, ever want to touch it, since
even a <1uF microwave oven capacitor will knock you quite dead.