Here is the concept of my design.
Use a lithium or nickel-metal hydride battery (preferably lithium) pack(s) that could maintain the lights for more than 20 hrs. full intensity using a halogen bulb for the headlight and LEDs for the tail just on a charge alone. The magneto would trickle charge the battery pack or provide supplemental power during full intensity use of the lights.
The head lamp that I have has four options, 1. full intensity halogen, 2. low intensity halogen, 3. continuous bright LEDs 4. flashing bright LEDs.
During daylight driving I could run the flashing LEDs both front and back from the power of the magneto, while still having enough current to provide a trickle charge to the battery pack and as an option be able to plug the charger into a wall outlet that could give absolute dead batteries a full charge in 1 to 3 hours.
It can be done. The trick here is to limit the amount of current to the charging system and allowing enough current for the ignition system. I'll have to get mine running and take some measurements to find out what the capabilities of the magneto and the demands of the ignition are before I can start putting it all together. I figure though that at idle speed the ignition is probably going to need everything that it can get from the magneto. I don't really want to have to make it a computer controlled system, but I may have to, just to make it work efficiently. That's yet to be seen, first I need to get mine running so I'll know what I'm dealing with.
I don't want to have to spend 20.00 a month on batteries and also have the chance of the batteries going dead on me during a night ride, when I'm 5 miles (or more) from home.