Couple of questions: why not just go full-wave rectified to DC to charge a standard moto cell battery, than tap the lights off of that?
I'm not familiar with a switching converter? Getting hits for buck converter, which may mean you're trying to shunt regulate/step-down?
I understand the intrigue, but I am just curious why your DC conversion isn't just a full bridge rec/reg that's an off-the-shelf part. And also why you need to run the lights with the bike off?