I dunno.
I think you are talking about 2 different things.
A resistor coil needs 7-9 volts for normal running. They always have a second wire so that they get 12 volts , or what is left while the starter is running.
Whether you run your ignition amplifier on 12 volts or 7-9 is a different issue. Most ignitions use 12 volts. Its irrelevant whether you take this from the coil input if it is 12 volts or somewhere else.

The trigger for the coil goes from the ignitioon to the NEGATIVE side of the coil , so as long as the coil load specs are correct, this has no direct connection with the POWER input of the ignition module.

I can tell you 100% that an 81 Range Rover has a RESISTOR WIRE from the ignition switch. This is the same as a ballast resisitor. YOU DO NOT NEED ANOTHER BALLAST RESISTOR for the coil.
The power to the ignition is a different issue. if you find 7-9 volts at the coil and the ignition specifies 7-9 volts INPUT then by all means take it from the coil input. If the spec is for 7-9 volts input which I doubt, then you must place a ballast resistor on the input of the ignition if you take the power from somehwhere else.
I am no electronic expert but I am pretty sure that with the very small load of a transistor ignition, the resistor will not lower the voltage as it needs a load. I am sure someone else will correct me if I am wrong on this point.
Regards Philip A