This can be done. A zener diode of 6 volts is what you need. I will have to draw up a diagram of the circuit and send it to you. You use the brake light coil to charge your battery because that coil isn't doing anything most of the time while you are ridding. The way I have the system set up is that you switch between the battery or your regular system to power your headlight and tail light. During daylight ridding you are charging your battery and then at night if you are doing a lot of stop and go driving you can switch over to the battery to give you a constant headlight. Nothing more annoying then pulling up to a stop sign and having the lights go out on ya.
If it ain't broke, you're not trying hard enough.
Lamborn's Miniature Engines