... to see if they can run a good halogen 12v car spotlight that lights up the way ahead on a moonless night in the outback.
Unfortunately, I don't see that happening. The kill switch works by shorting out the primary side of the coil. The A/C pulses of electricity which would normally be stepped up in voltage a few hundred times in the coil to create the spark get shunted to ground, and no spark is created. A standard headlight, halogen headlight, etc,. has a very low electrical resistance. (12V, 55W is .04 Ohm, and even
less until it heats up.) If you place a halogen headlight directly across the coil (which is what you would be doing) it would work just like a kill switch. It just sucks ALL the power away from the spark. Another way to think of it is that the magneto pickup coil is limited to a certain amount of power that it can provide. If you divert too much of it away from the spark plug, the plug won't 'spark,' and that pretty much defeats the entire purpose of the thing...
What folks have fould out by experimentation is that with white-wire systems, you can draw away about 3 watts of power before it seriously impacts the spark - before you have to reduce your spark gap to account for the reduced spark power that's available. Someone will have to test this with the black-wire system on the 4-strokes.
The tail light is pretty irrelevant because a battery LED does that for a very long time very well. It's the front end guys where the problem is and I doubt LEDs are good enough at todays state of technology...
Well, the new LEDs seem to do the job just fine. A 3-watt LED is awfully bright.
Now, what you might have to do is to decide what you can live with. Suppose you decide that you absolutely HAVE to have 6 watts of LED light. OK. What percentage of the time that you ride do you need this light? In my case, I commute to/from work. During the winter months, I would need lights about 50% of the time. If I could capture all the power from the black/white wire, and store it in a battery when I'm NOT using the light, that battery can then supplement the white/black wire at night, when the light is on. And, if I ran out of battery on an extended night run, I could always switch off one of the LEDs and run a little slower.
This design tradeoff is what I'm looking at with my dynohub approach.